diff --git a/amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/relays/RelayInformationScreen.kt b/amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/relays/RelayInformationScreen.kt
index 501c24ae0..375f0dd12 100644
--- a/amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/relays/RelayInformationScreen.kt
+++ b/amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/relays/RelayInformationScreen.kt
@@ -229,6 +229,8 @@ import com.vitorpamplona.quartz.nip64Chess.LiveChessGameChallengeEvent
 import com.vitorpamplona.quartz.nip64Chess.LiveChessGameEndEvent
 import com.vitorpamplona.quartz.nip64Chess.LiveChessMoveEvent
 import com.vitorpamplona.quartz.nip65RelayList.AdvertisedRelayListEvent
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.RelayDiscoveryEvent
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.RelayMonitorEvent
 import com.vitorpamplona.quartz.nip68Picture.PictureEvent
 import com.vitorpamplona.quartz.nip71Video.VideoHorizontalEvent
 import com.vitorpamplona.quartz.nip71Video.VideoNormalEvent
@@ -582,6 +584,8 @@ private fun kindDisplayName(kind: Int): Int =
         ReactionEvent.KIND -> R.string.kind_reactions
         ContactCardEvent.KIND -> R.string.kind_contact_card
         RelayAuthEvent.KIND -> R.string.kind_relay_auth
+        RelayDiscoveryEvent.KIND -> R.string.kind_relay_discovery
+        RelayMonitorEvent.KIND -> R.string.kind_relay_monitor
         RelaySetEvent.KIND -> R.string.kind_relay_set
         ReportEvent.KIND -> R.string.kind_reports
         RepostEvent.KIND -> R.string.kind_reposts
diff --git a/amethyst/src/main/res/values/strings.xml b/amethyst/src/main/res/values/strings.xml
index b115e2706..771a576c6 100644
--- a/amethyst/src/main/res/values/strings.xml
+++ b/amethyst/src/main/res/values/strings.xml
@@ -1663,6 +1663,8 @@
     <string name="kind_reactions">Reactions</string>
     <string name="kind_contact_card">Contact Card</string>
     <string name="kind_relay_auth">Relay Auth</string>
+    <string name="kind_relay_discovery">Relay Discovery</string>
+    <string name="kind_relay_monitor">Relay Monitor Announcement</string>
     <string name="kind_relay_set">Relay Set</string>
     <string name="kind_reports">Reports</string>
     <string name="kind_reposts">Reposts</string>
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/RelayDiscoveryEvent.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/RelayDiscoveryEvent.kt
new file mode 100644
index 000000000..560195443
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/RelayDiscoveryEvent.kt
@@ -0,0 +1,110 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.discovery
+
+import androidx.compose.runtime.Immutable
+import com.vitorpamplona.quartz.nip01Core.core.BaseAddressableEvent
+import com.vitorpamplona.quartz.nip01Core.core.HexKey
+import com.vitorpamplona.quartz.nip01Core.core.TagArrayBuilder
+import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
+import com.vitorpamplona.quartz.nip01Core.relay.normalizer.RelayUrlNormalizer
+import com.vitorpamplona.quartz.nip01Core.signers.eventTemplate
+import com.vitorpamplona.quartz.nip01Core.tags.dTag.dTag
+import com.vitorpamplona.quartz.nip01Core.tags.geohash.geohashes
+import com.vitorpamplona.quartz.nip31Alts.alt
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.RttType
+import com.vitorpamplona.quartz.utils.TimeUtils
+
+/**
+ * NIP-66 Kind 30166: Relay Discovery Event
+ *
+ * Published by relay monitors to document the state of a specific relay.
+ * The `d` tag holds the normalized relay URL, making each relay URL
+ * addressable per monitor pubkey.
+ *
+ * Carries metrics and metadata observed during monitoring:
+ * - Round-trip times (rtt-open, rtt-read, rtt-write)
+ * - Network type (n), relay type (T)
+ * - Supported NIPs (N), accepted kinds (k)
+ * - Requirements (R: auth, payment, pow, writes)
+ * - Topics (t) and geohash (g)
+ */
+@Immutable
+class RelayDiscoveryEvent(
+    id: HexKey,
+    pubKey: HexKey,
+    createdAt: Long,
+    tags: Array<Array<String>>,
+    content: String,
+    sig: HexKey,
+) : BaseAddressableEvent(id, pubKey, createdAt, KIND, tags, content, sig) {
+    fun relay(): NormalizedRelayUrl? = RelayUrlNormalizer.normalizeOrNull(dTag())
+
+    fun rtt() = tags.rtts()
+
+    fun rttOpen() = tags.rtt(RttType.OPEN)
+
+    fun rttRead() = tags.rtt(RttType.READ)
+
+    fun rttWrite() = tags.rtt(RttType.WRITE)
+
+    fun networkTypes() = tags.networkTypes()
+
+    fun relayTypes() = tags.relayTypes()
+
+    fun supportedNips() = tags.supportedNips()
+
+    fun requirements() = tags.requirements()
+
+    fun acceptedKinds() = tags.acceptedKinds()
+
+    fun topics() = tags.topics()
+
+    fun geohashes() = tags.geohashes()
+
+    companion object {
+        const val KIND = 30166
+        const val ALT_DESCRIPTION = "Relay discovery"
+
+        fun build(
+            relayUrl: NormalizedRelayUrl,
+            content: String = "",
+            createdAt: Long = TimeUtils.now(),
+            initializer: TagArrayBuilder<RelayDiscoveryEvent>.() -> Unit = {},
+        ) = eventTemplate<RelayDiscoveryEvent>(KIND, content, createdAt) {
+            alt(ALT_DESCRIPTION)
+            dTag(relayUrl.url)
+            initializer()
+        }
+
+        fun build(
+            relayUrl: String,
+            content: String = "",
+            createdAt: Long = TimeUtils.now(),
+            initializer: TagArrayBuilder<RelayDiscoveryEvent>.() -> Unit = {},
+        ) = build(
+            relayUrl = RelayUrlNormalizer.normalize(relayUrl),
+            content = content,
+            createdAt = createdAt,
+            initializer = initializer,
+        )
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/TagArrayBuilderExt.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/TagArrayBuilderExt.kt
new file mode 100644
index 000000000..73234032d
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/TagArrayBuilderExt.kt
@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.discovery
+
+import com.vitorpamplona.quartz.nip01Core.core.TagArrayBuilder
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.AcceptedKindTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.NetworkType
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.NetworkTypeTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.RelayTypeTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.RequirementTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.RttTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.RttType
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.SupportedNipTag
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.rtt(
+    type: RttType,
+    milliseconds: Long,
+) = addUnique(RttTag.assemble(type, milliseconds))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.networkType(network: NetworkType) = addUnique(NetworkTypeTag.assemble(network))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.networkTypes(networks: List<NetworkType>) = addAll(NetworkTypeTag.assemble(networks))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.relayType(type: String) = addUnique(RelayTypeTag.assemble(type))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.relayTypes(types: List<String>) = addAll(RelayTypeTag.assemble(types))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.supportedNip(nip: Int) = add(SupportedNipTag.assemble(nip))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.supportedNips(nips: List<Int>) = addAll(SupportedNipTag.assemble(nips))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.requirement(
+    value: String,
+    negated: Boolean = false,
+) = add(RequirementTag.assemble(value, negated))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.requirements(reqs: List<RequirementTag>) = addAll(RequirementTag.assemble(reqs))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.acceptedKind(
+    kind: Int,
+    negated: Boolean = false,
+) = add(AcceptedKindTag.assemble(kind, negated))
+
+fun TagArrayBuilder<RelayDiscoveryEvent>.acceptedKinds(kinds: List<AcceptedKindTag>) = addAll(AcceptedKindTag.assemble(kinds))
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/TagArrayExt.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/TagArrayExt.kt
new file mode 100644
index 000000000..a2944d40b
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/TagArrayExt.kt
@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.discovery
+
+import com.vitorpamplona.quartz.nip01Core.core.TagArray
+import com.vitorpamplona.quartz.nip01Core.tags.hashtags.hashtags
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.AcceptedKindTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.NetworkTypeTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.RelayTypeTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.RequirementTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.RttTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.RttType
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags.SupportedNipTag
+
+fun TagArray.rtts() = mapNotNull(RttTag::parse)
+
+fun TagArray.rtt(type: RttType) = mapNotNull(RttTag::parse).firstOrNull { it.type == type }?.milliseconds
+
+fun TagArray.networkTypes() = mapNotNull(NetworkTypeTag::parse)
+
+fun TagArray.relayTypes() = mapNotNull(RelayTypeTag::parse)
+
+fun TagArray.supportedNips() = mapNotNull(SupportedNipTag::parse)
+
+fun TagArray.requirements() = mapNotNull(RequirementTag::parse)
+
+fun TagArray.acceptedKinds() = mapNotNull(AcceptedKindTag::parse)
+
+fun TagArray.topics() = hashtags()
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/AcceptedKindTag.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/AcceptedKindTag.kt
new file mode 100644
index 000000000..a19e1d93e
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/AcceptedKindTag.kt
@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags
+
+import androidx.compose.runtime.Stable
+import com.vitorpamplona.quartz.nip01Core.core.has
+import com.vitorpamplona.quartz.utils.ensure
+
+/**
+ * ["k", "<kind>"] - an event kind accepted by this relay.
+ *
+ * A leading "!" negates, meaning the relay does NOT accept that kind.
+ * Examples: "1" (accepts kind 1), "!1" (rejects kind 1)
+ */
+@Stable
+class AcceptedKindTag(
+    val kind: Int,
+    val negated: Boolean,
+) {
+    companion object {
+        const val TAG_NAME = "k"
+        private const val NEGATION_PREFIX = "!"
+
+        fun isTag(tag: Array<String>) = tag.has(1) && tag[0] == TAG_NAME
+
+        fun parse(tag: Array<String>): AcceptedKindTag? {
+            ensure(tag.has(1)) { return null }
+            ensure(tag[0] == TAG_NAME) { return null }
+            ensure(tag[1].isNotEmpty()) { return null }
+            val raw = tag[1]
+            return if (raw.startsWith(NEGATION_PREFIX)) {
+                val kind = raw.removePrefix(NEGATION_PREFIX).toIntOrNull() ?: return null
+                AcceptedKindTag(kind = kind, negated = true)
+            } else {
+                val kind = raw.toIntOrNull() ?: return null
+                AcceptedKindTag(kind = kind, negated = false)
+            }
+        }
+
+        fun assemble(
+            kind: Int,
+            negated: Boolean = false,
+        ) = arrayOf(TAG_NAME, if (negated) "$NEGATION_PREFIX$kind" else kind.toString())
+
+        fun assemble(entry: AcceptedKindTag) = assemble(entry.kind, entry.negated)
+
+        fun assemble(kinds: List<AcceptedKindTag>) = kinds.map { assemble(it) }
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/NetworkTypeTag.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/NetworkTypeTag.kt
new file mode 100644
index 000000000..91d76e68e
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/NetworkTypeTag.kt
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags
+
+import com.vitorpamplona.quartz.nip01Core.core.has
+import com.vitorpamplona.quartz.utils.ensure
+
+/** Network accessibility types for relay connections */
+enum class NetworkType(
+    val code: String,
+) {
+    CLEARNET("clearnet"),
+    TOR("tor"),
+    I2P("i2p"),
+    LOKI("loki"),
+}
+
+/** ["n", "<network-type>"] - the network type the relay is accessible on */
+class NetworkTypeTag {
+    companion object {
+        const val TAG_NAME = "n"
+
+        fun isTag(tag: Array<String>) = tag.has(1) && tag[0] == TAG_NAME
+
+        fun parse(tag: Array<String>): NetworkType? {
+            ensure(tag.has(1)) { return null }
+            ensure(tag[0] == TAG_NAME) { return null }
+            ensure(tag[1].isNotEmpty()) { return null }
+            return NetworkType.entries.find { it.code == tag[1] }
+        }
+
+        fun assemble(network: NetworkType) = arrayOf(TAG_NAME, network.code)
+
+        fun assemble(networks: List<NetworkType>) = networks.map { assemble(it) }
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/RelayTypeTag.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/RelayTypeTag.kt
new file mode 100644
index 000000000..2e6f9b7ff
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/RelayTypeTag.kt
@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags
+
+import com.vitorpamplona.quartz.nip01Core.core.has
+import com.vitorpamplona.quartz.utils.ensure
+
+/**
+ * ["T", "<relay-type>"] - the type of relay in PascalCase (e.g. PrivateInbox, PublicOutbox).
+ * Values are arbitrary strings defined by relay operators; not an enum.
+ */
+class RelayTypeTag {
+    companion object {
+        const val TAG_NAME = "T"
+
+        fun isTag(tag: Array<String>) = tag.has(1) && tag[0] == TAG_NAME
+
+        fun parse(tag: Array<String>): String? {
+            ensure(tag.has(1)) { return null }
+            ensure(tag[0] == TAG_NAME) { return null }
+            ensure(tag[1].isNotEmpty()) { return null }
+            return tag[1]
+        }
+
+        fun assemble(relayType: String) = arrayOf(TAG_NAME, relayType)
+
+        fun assemble(relayTypes: List<String>) = relayTypes.map { assemble(it) }
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/RequirementTag.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/RequirementTag.kt
new file mode 100644
index 000000000..4815bccbd
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/RequirementTag.kt
@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags
+
+import androidx.compose.runtime.Stable
+import com.vitorpamplona.quartz.nip01Core.core.has
+import com.vitorpamplona.quartz.utils.ensure
+
+/**
+ * ["R", "<requirement>"] - access requirement enforced by the relay.
+ *
+ * A leading "!" negates the requirement, meaning the relay does NOT enforce it.
+ * Examples: "auth", "payment", "!payment" (no payment required), "pow", "writes"
+ */
+@Stable
+class RequirementTag(
+    val value: String,
+    val negated: Boolean,
+) {
+    companion object {
+        const val TAG_NAME = "R"
+        private const val NEGATION_PREFIX = "!"
+
+        fun isTag(tag: Array<String>) = tag.has(1) && tag[0] == TAG_NAME
+
+        fun parse(tag: Array<String>): RequirementTag? {
+            ensure(tag.has(1)) { return null }
+            ensure(tag[0] == TAG_NAME) { return null }
+            ensure(tag[1].isNotEmpty()) { return null }
+            val raw = tag[1]
+            return if (raw.startsWith(NEGATION_PREFIX)) {
+                RequirementTag(value = raw.removePrefix(NEGATION_PREFIX), negated = true)
+            } else {
+                RequirementTag(value = raw, negated = false)
+            }
+        }
+
+        fun assemble(
+            value: String,
+            negated: Boolean = false,
+        ) = arrayOf(TAG_NAME, if (negated) "$NEGATION_PREFIX$value" else value)
+
+        fun assemble(req: RequirementTag) = assemble(req.value, req.negated)
+
+        fun assemble(requirements: List<RequirementTag>) = requirements.map { assemble(it) }
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/RttTag.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/RttTag.kt
new file mode 100644
index 000000000..534d2ab48
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/RttTag.kt
@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags
+
+import androidx.compose.runtime.Stable
+import com.vitorpamplona.quartz.nip01Core.core.has
+import com.vitorpamplona.quartz.utils.ensure
+
+/** Round-trip time measurement types */
+enum class RttType(
+    val tagName: String,
+) {
+    OPEN("rtt-open"),
+    READ("rtt-read"),
+    WRITE("rtt-write"),
+}
+
+/**
+ * Round-trip time tags measured in milliseconds.
+ *
+ * ["rtt-open", "<milliseconds>"]
+ * ["rtt-read", "<milliseconds>"]
+ * ["rtt-write", "<milliseconds>"]
+ */
+@Stable
+class RttTag(
+    val type: RttType,
+    val milliseconds: Long,
+) {
+    companion object {
+        fun isTag(tag: Array<String>) = tag.has(1) && RttType.entries.any { it.tagName == tag[0] }
+
+        fun parse(tag: Array<String>): RttTag? {
+            ensure(tag.has(1)) { return null }
+            val type = RttType.entries.find { it.tagName == tag[0] } ?: return null
+            val ms = tag[1].toLongOrNull() ?: return null
+            return RttTag(type, ms)
+        }
+
+        fun assemble(
+            type: RttType,
+            milliseconds: Long,
+        ) = arrayOf(type.tagName, milliseconds.toString())
+
+        fun assemble(rtt: RttTag) = assemble(rtt.type, rtt.milliseconds)
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/SupportedNipTag.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/SupportedNipTag.kt
new file mode 100644
index 000000000..6cfe3ba87
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/discovery/tags/SupportedNipTag.kt
@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.discovery.tags
+
+import com.vitorpamplona.quartz.nip01Core.core.has
+import com.vitorpamplona.quartz.utils.ensure
+
+/** ["N", "<nip-number>"] - a NIP number supported by this relay (repeated per supported NIP) */
+class SupportedNipTag {
+    companion object {
+        const val TAG_NAME = "N"
+
+        fun isTag(tag: Array<String>) = tag.has(1) && tag[0] == TAG_NAME
+
+        fun parse(tag: Array<String>): Int? {
+            ensure(tag.has(1)) { return null }
+            ensure(tag[0] == TAG_NAME) { return null }
+            ensure(tag[1].isNotEmpty()) { return null }
+            return tag[1].toIntOrNull()
+        }
+
+        fun assemble(nip: Int) = arrayOf(TAG_NAME, nip.toString())
+
+        fun assemble(nips: List<Int>) = nips.map { assemble(it) }
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/RelayMonitorEvent.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/RelayMonitorEvent.kt
new file mode 100644
index 000000000..a84925eb3
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/RelayMonitorEvent.kt
@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.monitor
+
+import androidx.compose.runtime.Immutable
+import com.vitorpamplona.quartz.nip01Core.core.BaseReplaceableEvent
+import com.vitorpamplona.quartz.nip01Core.core.HexKey
+import com.vitorpamplona.quartz.nip01Core.core.TagArrayBuilder
+import com.vitorpamplona.quartz.nip01Core.signers.eventTemplate
+import com.vitorpamplona.quartz.nip01Core.tags.geohash.geohash
+import com.vitorpamplona.quartz.nip01Core.tags.geohash.geohashes
+import com.vitorpamplona.quartz.nip31Alts.alt
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags.CheckType
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags.TimeoutTag
+import com.vitorpamplona.quartz.utils.TimeUtils
+
+/**
+ * NIP-66 Kind 10166: Relay Monitor Announcement
+ *
+ * Published by monitoring services to advertise themselves. Declares:
+ * - How frequently the monitor publishes results (frequency tag, required)
+ * - Which types of checks it performs (c tags)
+ * - Timeout values used for each check type (timeout tags)
+ * - Geographical location of the monitor (g tags)
+ *
+ * Monitors should also publish complementary kind 0 (metadata) and
+ * kind 10002 (relay list) events for full discoverability.
+ */
+@Immutable
+class RelayMonitorEvent(
+    id: HexKey,
+    pubKey: HexKey,
+    createdAt: Long,
+    tags: Array<Array<String>>,
+    content: String,
+    sig: HexKey,
+) : BaseReplaceableEvent(id, pubKey, createdAt, KIND, tags, content, sig) {
+    fun frequency() = tags.frequency()
+
+    fun checks() = tags.checks()
+
+    fun timeouts() = tags.timeouts()
+
+    fun geohashes() = tags.geohashes()
+
+    companion object {
+        const val KIND = 10166
+        const val ALT_DESCRIPTION = "Relay monitor announcement"
+
+        fun build(
+            frequencySeconds: Long,
+            checks: List<CheckType> = emptyList(),
+            timeouts: List<TimeoutTag> = emptyList(),
+            geohash: String? = null,
+            createdAt: Long = TimeUtils.now(),
+            initializer: TagArrayBuilder<RelayMonitorEvent>.() -> Unit = {},
+        ) = eventTemplate<RelayMonitorEvent>(KIND, "", createdAt) {
+            alt(ALT_DESCRIPTION)
+            frequency(frequencySeconds)
+            checks(checks)
+            timeouts(timeouts)
+            geohash?.let { geohash(it) }
+            initializer()
+        }
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/TagArrayBuilderExt.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/TagArrayBuilderExt.kt
new file mode 100644
index 000000000..ba2d64568
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/TagArrayBuilderExt.kt
@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.monitor
+
+import com.vitorpamplona.quartz.nip01Core.core.TagArrayBuilder
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags.CheckTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags.CheckType
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags.FrequencyTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags.TimeoutTag
+
+fun TagArrayBuilder<RelayMonitorEvent>.frequency(seconds: Long) = addUnique(FrequencyTag.assemble(seconds))
+
+fun TagArrayBuilder<RelayMonitorEvent>.checks(checks: List<CheckType>) = addAll(CheckTag.assemble(checks))
+
+fun TagArrayBuilder<RelayMonitorEvent>.timeouts(timeouts: List<TimeoutTag>) = addAll(TimeoutTag.assemble(timeouts))
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/TagArrayExt.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/TagArrayExt.kt
new file mode 100644
index 000000000..ee507b330
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/TagArrayExt.kt
@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.monitor
+
+import com.vitorpamplona.quartz.nip01Core.core.TagArray
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags.CheckTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags.FrequencyTag
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags.TimeoutTag
+
+fun TagArray.frequency() = firstNotNullOfOrNull(FrequencyTag::parse)
+
+fun TagArray.checks() = mapNotNull(CheckTag::parse)
+
+fun TagArray.timeouts() = mapNotNull(TimeoutTag::parse)
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/tags/CheckTag.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/tags/CheckTag.kt
new file mode 100644
index 000000000..2db69a314
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/tags/CheckTag.kt
@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags
+
+import com.vitorpamplona.quartz.nip01Core.core.has
+import com.vitorpamplona.quartz.utils.ensure
+
+/** Types of checks a relay monitor can perform */
+enum class CheckType(
+    val code: String,
+) {
+    OPEN("open"),
+    READ("read"),
+    WRITE("write"),
+    AUTH("auth"),
+    NIP11("nip11"),
+    DNS("dns"),
+    GEO("geo"),
+}
+
+/** ["c", "<check-type>"] - a capability check conducted by this monitor */
+class CheckTag {
+    companion object {
+        const val TAG_NAME = "c"
+
+        fun isTag(tag: Array<String>) = tag.has(1) && tag[0] == TAG_NAME
+
+        fun parse(tag: Array<String>): CheckType? {
+            ensure(tag.has(1)) { return null }
+            ensure(tag[0] == TAG_NAME) { return null }
+            ensure(tag[1].isNotEmpty()) { return null }
+            return CheckType.entries.find { it.code == tag[1] }
+        }
+
+        fun assemble(check: CheckType) = arrayOf(TAG_NAME, check.code)
+
+        fun assemble(checks: List<CheckType>) = checks.map { assemble(it) }
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/tags/FrequencyTag.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/tags/FrequencyTag.kt
new file mode 100644
index 000000000..95c6d7d1a
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/tags/FrequencyTag.kt
@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags
+
+import com.vitorpamplona.quartz.nip01Core.core.has
+import com.vitorpamplona.quartz.utils.ensure
+
+/** ["frequency", "<seconds>"] - how often this monitor publishes check results */
+class FrequencyTag {
+    companion object {
+        const val TAG_NAME = "frequency"
+
+        fun isTag(tag: Array<String>) = tag.has(1) && tag[0] == TAG_NAME
+
+        fun parse(tag: Array<String>): Long? {
+            ensure(tag.has(1)) { return null }
+            ensure(tag[0] == TAG_NAME) { return null }
+            ensure(tag[1].isNotEmpty()) { return null }
+            return tag[1].toLongOrNull()
+        }
+
+        fun assemble(seconds: Long) = arrayOf(TAG_NAME, seconds.toString())
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/tags/TimeoutTag.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/tags/TimeoutTag.kt
new file mode 100644
index 000000000..da9a752a3
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip66RelayMonitor/monitor/tags/TimeoutTag.kt
@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip66RelayMonitor.monitor.tags
+
+import androidx.compose.runtime.Stable
+import com.vitorpamplona.quartz.nip01Core.core.has
+import com.vitorpamplona.quartz.utils.ensure
+
+/**
+ * Timeout tag for relay monitors.
+ *
+ * General form:  ["timeout", "<milliseconds>"]
+ * Per-check form: ["timeout", "<check-type>", "<milliseconds>"]
+ *
+ * Where check-type is one of: open, read, write, nip11
+ */
+@Stable
+class TimeoutTag(
+    val checkType: String?,
+    val milliseconds: Long,
+) {
+    companion object {
+        const val TAG_NAME = "timeout"
+
+        fun isTag(tag: Array<String>) = tag.has(1) && tag[0] == TAG_NAME
+
+        fun parse(tag: Array<String>): TimeoutTag? {
+            ensure(tag.has(1)) { return null }
+            ensure(tag[0] == TAG_NAME) { return null }
+            return if (tag.has(2)) {
+                val ms = tag[2].toLongOrNull() ?: return null
+                TimeoutTag(checkType = tag[1], milliseconds = ms)
+            } else {
+                val ms = tag[1].toLongOrNull() ?: return null
+                TimeoutTag(checkType = null, milliseconds = ms)
+            }
+        }
+
+        fun assemble(milliseconds: Long) = arrayOf(TAG_NAME, milliseconds.toString())
+
+        fun assemble(
+            checkType: String,
+            milliseconds: Long,
+        ) = arrayOf(TAG_NAME, checkType, milliseconds.toString())
+
+        fun assemble(timeout: TimeoutTag) =
+            if (timeout.checkType != null) {
+                assemble(timeout.checkType, timeout.milliseconds)
+            } else {
+                assemble(timeout.milliseconds)
+            }
+
+        fun assemble(timeouts: List<TimeoutTag>) = timeouts.map { assemble(it) }
+    }
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/EventFactory.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/EventFactory.kt
index c570731f9..2870e06fa 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/EventFactory.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/EventFactory.kt
@@ -121,6 +121,8 @@ import com.vitorpamplona.quartz.nip64Chess.LiveChessGameChallengeEvent
 import com.vitorpamplona.quartz.nip64Chess.LiveChessGameEndEvent
 import com.vitorpamplona.quartz.nip64Chess.LiveChessMoveEvent
 import com.vitorpamplona.quartz.nip65RelayList.AdvertisedRelayListEvent
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.RelayDiscoveryEvent
+import com.vitorpamplona.quartz.nip66RelayMonitor.monitor.RelayMonitorEvent
 import com.vitorpamplona.quartz.nip68Picture.PictureEvent
 import com.vitorpamplona.quartz.nip71Video.VideoHorizontalEvent
 import com.vitorpamplona.quartz.nip71Video.VideoNormalEvent
@@ -281,6 +283,8 @@ class EventFactory {
                 ReactionEvent.KIND -> ReactionEvent(id, pubKey, createdAt, tags, content, sig)
                 ContactCardEvent.KIND -> ContactCardEvent(id, pubKey, createdAt, tags, content, sig)
                 RelayAuthEvent.KIND -> RelayAuthEvent(id, pubKey, createdAt, tags, content, sig)
+                RelayDiscoveryEvent.KIND -> RelayDiscoveryEvent(id, pubKey, createdAt, tags, content, sig)
+                RelayMonitorEvent.KIND -> RelayMonitorEvent(id, pubKey, createdAt, tags, content, sig)
                 RelaySetEvent.KIND -> RelaySetEvent(id, pubKey, createdAt, tags, content, sig)
                 ReportEvent.KIND -> ReportEvent(id, pubKey, createdAt, tags, content, sig)
                 RepostEvent.KIND -> RepostEvent(id, pubKey, createdAt, tags, content, sig)