diff --git a/crates/application/src/lib.rs b/crates/application/src/lib.rs
index d07542b..05db0a8 100644
--- a/crates/application/src/lib.rs
+++ b/crates/application/src/lib.rs
@@ -1 +1,2 @@
+pub mod services;
 pub mod use_cases;
diff --git a/crates/application/src/services/mod.rs b/crates/application/src/services/mod.rs
new file mode 100644
index 0000000..480fdc6
--- /dev/null
+++ b/crates/application/src/services/mod.rs
@@ -0,0 +1,2 @@
+pub mod notification_event;
+pub use notification_event::NotificationEventService;
diff --git a/crates/application/src/services/notification_event.rs b/crates/application/src/services/notification_event.rs
new file mode 100644
index 0000000..6747f06
--- /dev/null
+++ b/crates/application/src/services/notification_event.rs
@@ -0,0 +1,217 @@
+use std::sync::Arc;
+use chrono::Utc;
+use domain::{
+    errors::DomainError,
+    events::DomainEvent,
+    models::notification::{Notification, NotificationType},
+    ports::{NotificationRepository, ThoughtRepository},
+    value_objects::NotificationId,
+};
+
+pub struct NotificationEventService {
+    pub thoughts:      Arc<dyn ThoughtRepository>,
+    pub notifications: Arc<dyn NotificationRepository>,
+}
+
+impl NotificationEventService {
+    pub async fn process(&self, event: &DomainEvent) -> Result<(), DomainError> {
+        match event {
+            DomainEvent::LikeAdded { like_id: _, user_id, thought_id } => {
+                let thought = match self.thoughts.find_by_id(thought_id).await? {
+                    Some(t) => t,
+                    None => return Ok(()),
+                };
+                if thought.user_id == *user_id { return Ok(()); }
+                self.notifications.save(&Notification {
+                    id: NotificationId::new(),
+                    user_id: thought.user_id,
+                    notification_type: NotificationType::Like,
+                    from_user_id: Some(user_id.clone()),
+                    thought_id: Some(thought_id.clone()),
+                    read: false,
+                    created_at: Utc::now(),
+                }).await
+            }
+            DomainEvent::BoostAdded { boost_id: _, user_id, thought_id } => {
+                let thought = match self.thoughts.find_by_id(thought_id).await? {
+                    Some(t) => t,
+                    None => return Ok(()),
+                };
+                if thought.user_id == *user_id { return Ok(()); }
+                self.notifications.save(&Notification {
+                    id: NotificationId::new(),
+                    user_id: thought.user_id,
+                    notification_type: NotificationType::Boost,
+                    from_user_id: Some(user_id.clone()),
+                    thought_id: Some(thought_id.clone()),
+                    read: false,
+                    created_at: Utc::now(),
+                }).await
+            }
+            DomainEvent::FollowAccepted { follower_id, following_id } => {
+                self.notifications.save(&Notification {
+                    id: NotificationId::new(),
+                    user_id: following_id.clone(),
+                    notification_type: NotificationType::Follow,
+                    from_user_id: Some(follower_id.clone()),
+                    thought_id: None,
+                    read: false,
+                    created_at: Utc::now(),
+                }).await
+            }
+            DomainEvent::ThoughtCreated { thought_id, user_id, in_reply_to_id } => {
+                let reply_to_id = match in_reply_to_id {
+                    Some(id) => id,
+                    None => return Ok(()),
+                };
+                let original = match self.thoughts.find_by_id(reply_to_id).await? {
+                    Some(t) => t,
+                    None => return Ok(()),
+                };
+                if original.user_id == *user_id { return Ok(()); }
+                self.notifications.save(&Notification {
+                    id: NotificationId::new(),
+                    user_id: original.user_id,
+                    notification_type: NotificationType::Reply,
+                    from_user_id: Some(user_id.clone()),
+                    thought_id: Some(thought_id.clone()),
+                    read: false,
+                    created_at: Utc::now(),
+                }).await
+            }
+            _ => Ok(()),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use domain::{
+        models::{thought::{Thought, Visibility}, user::User},
+        testing::TestStore,
+        value_objects::*,
+    };
+    use std::sync::Arc;
+
+    fn alice() -> User {
+        User::new_local(
+            UserId::new(),
+            Username::new("alice").unwrap(),
+            Email::new("alice@ex.com").unwrap(),
+            PasswordHash("h".into()),
+        )
+    }
+
+    #[tokio::test]
+    async fn like_creates_notification_for_thought_author() {
+        let store = TestStore::default();
+        let alice = alice();
+        let bob_id = UserId::new();
+        let thought = Thought::new_local(
+            ThoughtId::new(), alice.id.clone(),
+            Content::new_local("hello").unwrap(),
+            None, Visibility::Public, None, false,
+        );
+        store.thoughts.lock().unwrap().push(thought.clone());
+        let svc = NotificationEventService {
+            thoughts: Arc::new(store.clone()),
+            notifications: Arc::new(store.clone()),
+        };
+        svc.process(&DomainEvent::LikeAdded {
+            like_id: LikeId::new(),
+            user_id: bob_id,
+            thought_id: thought.id.clone(),
+        }).await.unwrap();
+        let notifs = store.notifications.lock().unwrap();
+        assert_eq!(notifs.len(), 1);
+        assert!(matches!(notifs[0].notification_type, NotificationType::Like));
+    }
+
+    #[tokio::test]
+    async fn self_like_creates_no_notification() {
+        let store = TestStore::default();
+        let alice = alice();
+        let thought = Thought::new_local(
+            ThoughtId::new(), alice.id.clone(),
+            Content::new_local("hello").unwrap(),
+            None, Visibility::Public, None, false,
+        );
+        store.thoughts.lock().unwrap().push(thought.clone());
+        let svc = NotificationEventService {
+            thoughts: Arc::new(store.clone()),
+            notifications: Arc::new(store.clone()),
+        };
+        svc.process(&DomainEvent::LikeAdded {
+            like_id: LikeId::new(),
+            user_id: alice.id.clone(),
+            thought_id: thought.id.clone(),
+        }).await.unwrap();
+        assert!(store.notifications.lock().unwrap().is_empty());
+    }
+
+    #[tokio::test]
+    async fn follow_accepted_creates_notification() {
+        let store = TestStore::default();
+        let alice = alice();
+        let bob_id = UserId::new();
+        let svc = NotificationEventService {
+            thoughts: Arc::new(store.clone()),
+            notifications: Arc::new(store.clone()),
+        };
+        svc.process(&DomainEvent::FollowAccepted {
+            follower_id: bob_id,
+            following_id: alice.id.clone(),
+        }).await.unwrap();
+        let notifs = store.notifications.lock().unwrap();
+        assert_eq!(notifs.len(), 1);
+        assert!(matches!(notifs[0].notification_type, NotificationType::Follow));
+    }
+
+    #[tokio::test]
+    async fn reply_creates_notification_for_original_author() {
+        let store = TestStore::default();
+        let alice = alice();
+        let bob_id = UserId::new();
+        let original = Thought::new_local(
+            ThoughtId::new(), alice.id.clone(),
+            Content::new_local("original").unwrap(),
+            None, Visibility::Public, None, false,
+        );
+        store.thoughts.lock().unwrap().push(original.clone());
+        let svc = NotificationEventService {
+            thoughts: Arc::new(store.clone()),
+            notifications: Arc::new(store.clone()),
+        };
+        svc.process(&DomainEvent::ThoughtCreated {
+            thought_id: ThoughtId::new(),
+            user_id: bob_id,
+            in_reply_to_id: Some(original.id.clone()),
+        }).await.unwrap();
+        let notifs = store.notifications.lock().unwrap();
+        assert_eq!(notifs.len(), 1);
+        assert!(matches!(notifs[0].notification_type, NotificationType::Reply));
+    }
+
+    #[tokio::test]
+    async fn self_reply_creates_no_notification() {
+        let store = TestStore::default();
+        let alice = alice();
+        let original = Thought::new_local(
+            ThoughtId::new(), alice.id.clone(),
+            Content::new_local("original").unwrap(),
+            None, Visibility::Public, None, false,
+        );
+        store.thoughts.lock().unwrap().push(original.clone());
+        let svc = NotificationEventService {
+            thoughts: Arc::new(store.clone()),
+            notifications: Arc::new(store.clone()),
+        };
+        svc.process(&DomainEvent::ThoughtCreated {
+            thought_id: ThoughtId::new(),
+            user_id: alice.id.clone(),
+            in_reply_to_id: Some(original.id.clone()),
+        }).await.unwrap();
+        assert!(store.notifications.lock().unwrap().is_empty());
+    }
+}