diff --git a/README.md b/README.md
index 93cc19f..c2abc44 100644
--- a/README.md
+++ b/README.md
@@ -40,7 +40,7 @@ The following [NIPs](https://github.com/nostr-protocol/nips) are implemented:
 - [ ] [NIP-28: Public Chat](https://github.com/nostr-protocol/nips/blob/master/28.md)
 - [ ] [NIP-29: Relay-based Groups](https://github.com/nostr-protocol/nips/blob/master/29.md)
 - [x] [NIP-30: Custom Emoji](https://github.com/nostr-protocol/nips/blob/master/30.md)
-- [ ] [NIP-31: Dealing with Unknown Events](https://github.com/nostr-protocol/nips/blob/master/31.md)
+- [x] [NIP-31: Dealing with Unknown Events](https://github.com/nostr-protocol/nips/blob/master/31.md)
 - [ ] [NIP-32: Labeling](https://github.com/nostr-protocol/nips/blob/master/32.md)
 - [ ] [NIP-34: `git` stuff](https://github.com/nostr-protocol/nips/blob/master/34.md)
 - [ ] [NIP-35: Torrents](https://github.com/nostr-protocol/nips/blob/master/35.md)
diff --git a/Sources/NostrSDK/Events/NostrEvent.swift b/Sources/NostrSDK/Events/NostrEvent.swift
index d1edbd3..6c6341a 100644
--- a/Sources/NostrSDK/Events/NostrEvent.swift
+++ b/Sources/NostrSDK/Events/NostrEvent.swift
@@ -150,6 +150,13 @@ public class NostrEvent: Codable, Equatable, Hashable {
         tags.compactMap { EventCoordinates(eventCoordinatesTag: $0) }
     }
 
+    /// A short human-readable plaintext summary of what the event is about
+    /// when the event kind is part of a custom protocol and isn't meant to be read as text (like kind:1).
+    /// See [NIP-31 - Dealing with unknown event kinds](https://github.com/nostr-protocol/nips/blob/master/31.md).
+    public var alternativeSummary: String? {
+        firstValueForTagName(.alternativeSummary)
+    }
+
     /// Unix timestamp at which the message SHOULD be considered expired (by relays and clients) and SHOULD be deleted by relays.
     /// See [NIP-40 - Expiration Timestamp](https://github.com/nostr-protocol/nips/blob/master/40.md).
     public var expiration: Int64? {
@@ -375,6 +382,15 @@ public extension NostrEvent {
             return self
         }
 
+        /// Specifies a short human-readable plaintext summary of what the event is about
+        /// when the event kind is part of a custom protocol and isn't meant to be read as text (like kind:1).
+        /// See [NIP-31 - Dealing with unknown event kinds](https://github.com/nostr-protocol/nips/blob/master/31.md).
+        @discardableResult
+        public final func alternativeSummary(_ alternativeSummary: String) -> Self {
+            tags.append(Tag(name: .alternativeSummary, value: alternativeSummary))
+            return self
+        }
+
         /// Specifies a unix timestamp at which the message SHOULD be considered expired (by relays and clients) and SHOULD be deleted by relays.
         /// See [NIP-40 - Expiration Timestamp](https://github.com/nostr-protocol/nips/blob/master/40.md).
         @discardableResult
diff --git a/Sources/NostrSDK/Tag.swift b/Sources/NostrSDK/Tag.swift
index 3af19c5..6c436f1 100644
--- a/Sources/NostrSDK/Tag.swift
+++ b/Sources/NostrSDK/Tag.swift
@@ -9,7 +9,12 @@ import Foundation
 
 /// A constant that describes the type of a ``Tag``.
 public enum TagName: String {
-    
+
+    /// A short human-readable plaintext summary of what the event is about
+    /// when the event kind is part of a custom protocol and isn't meant to be read as text (like kind:1).
+    /// See [NIP-31 - Dealing with unknown event kinds](https://github.com/nostr-protocol/nips/blob/master/31.md).
+    case alternativeSummary = "alt"
+
     /// a custom emoji that defines the shortcode name and image URL of the image file
     case emoji
     
diff --git a/Tests/NostrSDKTests/Events/NostrEventTests.swift b/Tests/NostrSDKTests/Events/NostrEventTests.swift
index 1527d8f..c6f1509 100644
--- a/Tests/NostrSDKTests/Events/NostrEventTests.swift
+++ b/Tests/NostrSDKTests/Events/NostrEventTests.swift
@@ -68,6 +68,17 @@ final class NostrEventTests: XCTestCase, FixtureLoading, MetadataCoding {
         XCTAssertEqual(metadata.relays?[1], relay2)
     }
 
+    func testAlternativeSummary() throws {
+        let alternativeSummary = "Alternative summary to display for clients that do not support this event kind."
+        let customEvent = try NostrEvent.Builder(kind: EventKind(rawValue: 23456))
+            .alternativeSummary(alternativeSummary)
+            .build(signedBy: .test)
+        XCTAssertEqual(customEvent.alternativeSummary, alternativeSummary)
+
+        let decodedCustomEventWithAltTag: NostrEvent = try decodeFixture(filename: "custom_event_alt_tag")
+        XCTAssertEqual(decodedCustomEventWithAltTag.alternativeSummary, alternativeSummary)
+    }
+
     func testExpiration() throws {
         let futureExpiration = Int64(Date.now.timeIntervalSince1970 + 10000)
         let futureExpirationEvent = try NostrEvent.Builder(kind: .textNote)
diff --git a/Tests/NostrSDKTests/Fixtures/custom_event_alt_tag.json b/Tests/NostrSDKTests/Fixtures/custom_event_alt_tag.json
new file mode 100644
index 0000000..5a5361b
--- /dev/null
+++ b/Tests/NostrSDKTests/Fixtures/custom_event_alt_tag.json
@@ -0,0 +1,9 @@
+{
+  "content": "",
+  "created_at": 1687090842,
+  "id": "b7fbbb4d473db3b3a671ad995f1248d56e9a2ed7f145c1e5f74308e60db2eba7",
+  "kind": 23456,
+  "pubkey": "9947f9659dd80c3682402b612f5447e28249997fb3709500c32a585eb0977340",
+  "sig": "09a1dcace3cec8cd9934b544cede6955c9876e385d73491f1ca5354a0baecda834c43ac9510ef779f8c8c7543b3faccd821e40e0a52dde8527d071cb0d53230c",
+  "tags": [["alt","Alternative summary to display for clients that do not support this event kind."]]
+}