diff --git a/crates/domain/src/events.rs b/crates/domain/src/events.rs
new file mode 100644
index 0000000..729d0f1
--- /dev/null
+++ b/crates/domain/src/events.rs
@@ -0,0 +1 @@
+// filled in Task 4
diff --git a/crates/domain/src/lib.rs b/crates/domain/src/lib.rs
index fa5d165..af60305 100644
--- a/crates/domain/src/lib.rs
+++ b/crates/domain/src/lib.rs
@@ -1,3 +1,8 @@
 pub mod errors;
+pub mod events;
+pub mod models;
+pub mod ports;
 pub mod value_objects;
-// remaining modules added in later tasks
+
+#[cfg(any(test, feature = "test-helpers"))]
+pub mod testing;
diff --git a/crates/domain/src/models/api_key.rs b/crates/domain/src/models/api_key.rs
new file mode 100644
index 0000000..101029c
--- /dev/null
+++ b/crates/domain/src/models/api_key.rs
@@ -0,0 +1,11 @@
+use chrono::{DateTime, Utc};
+use crate::value_objects::{ApiKeyId, UserId};
+
+#[derive(Debug, Clone)]
+pub struct ApiKey {
+    pub id: ApiKeyId,
+    pub user_id: UserId,
+    pub key_hash: String,
+    pub name: String,
+    pub created_at: DateTime<Utc>,
+}
diff --git a/crates/domain/src/models/feed.rs b/crates/domain/src/models/feed.rs
new file mode 100644
index 0000000..8cc226f
--- /dev/null
+++ b/crates/domain/src/models/feed.rs
@@ -0,0 +1,40 @@
+use crate::models::{user::User, thought::Thought};
+use crate::value_objects::UserId;
+
+#[derive(Debug, Clone)]
+pub struct UserSummary {
+    pub id: UserId,
+    pub username: String,
+    pub display_name: Option<String>,
+    pub avatar_url: Option<String>,
+    pub bio: Option<String>,
+    pub thought_count: i64,
+    pub follower_count: i64,
+    pub following_count: i64,
+}
+
+#[derive(Debug, Clone)]
+pub struct FeedEntry {
+    pub thought: Thought,
+    pub author: User,
+    pub like_count: i64,
+    pub boost_count: i64,
+    pub reply_count: i64,
+    pub liked_by_viewer: bool,
+    pub boosted_by_viewer: bool,
+}
+
+#[derive(Debug, Clone)]
+pub struct PageParams { pub page: u64, pub per_page: u64 }
+impl PageParams {
+    pub fn offset(&self) -> i64 { ((self.page.saturating_sub(1)) * self.per_page) as i64 }
+    pub fn limit(&self) -> i64 { self.per_page as i64 }
+}
+
+#[derive(Debug, Clone)]
+pub struct Paginated<T> {
+    pub items: Vec<T>,
+    pub total: i64,
+    pub page: u64,
+    pub per_page: u64,
+}
diff --git a/crates/domain/src/models/mod.rs b/crates/domain/src/models/mod.rs
new file mode 100644
index 0000000..bb56c47
--- /dev/null
+++ b/crates/domain/src/models/mod.rs
@@ -0,0 +1,9 @@
+pub mod api_key;
+pub mod feed;
+pub mod notification;
+pub mod remote_actor;
+pub mod social;
+pub mod tag;
+pub mod thought;
+pub mod top_friend;
+pub mod user;
diff --git a/crates/domain/src/models/notification.rs b/crates/domain/src/models/notification.rs
new file mode 100644
index 0000000..6db91b8
--- /dev/null
+++ b/crates/domain/src/models/notification.rs
@@ -0,0 +1,24 @@
+use chrono::{DateTime, Utc};
+use crate::value_objects::{NotificationId, UserId, ThoughtId};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum NotificationType { Like, Boost, Follow, Mention, Reply }
+impl NotificationType {
+    pub fn from_str(s: &str) -> Self {
+        match s { "like" => Self::Like, "boost" => Self::Boost, "follow" => Self::Follow, "mention" => Self::Mention, _ => Self::Reply }
+    }
+    pub fn as_str(&self) -> &str {
+        match self { Self::Like => "like", Self::Boost => "boost", Self::Follow => "follow", Self::Mention => "mention", Self::Reply => "reply" }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct Notification {
+    pub id: NotificationId,
+    pub user_id: UserId,
+    pub notification_type: NotificationType,
+    pub from_user_id: Option<UserId>,
+    pub thought_id: Option<ThoughtId>,
+    pub read: bool,
+    pub created_at: DateTime<Utc>,
+}
diff --git a/crates/domain/src/models/remote_actor.rs b/crates/domain/src/models/remote_actor.rs
new file mode 100644
index 0000000..f8d439c
--- /dev/null
+++ b/crates/domain/src/models/remote_actor.rs
@@ -0,0 +1,12 @@
+use chrono::{DateTime, Utc};
+
+#[derive(Debug, Clone)]
+pub struct RemoteActor {
+    pub url: String,
+    pub handle: String,
+    pub display_name: Option<String>,
+    pub inbox_url: String,
+    pub shared_inbox_url: Option<String>,
+    pub public_key: String,
+    pub last_fetched_at: DateTime<Utc>,
+}
diff --git a/crates/domain/src/models/social.rs b/crates/domain/src/models/social.rs
new file mode 100644
index 0000000..82cb53d
--- /dev/null
+++ b/crates/domain/src/models/social.rs
@@ -0,0 +1,47 @@
+use chrono::{DateTime, Utc};
+use crate::value_objects::{UserId, ThoughtId, LikeId, BoostId};
+
+#[derive(Debug, Clone)]
+pub struct Like {
+    pub id: LikeId,
+    pub user_id: UserId,
+    pub thought_id: ThoughtId,
+    pub ap_id: Option<String>,
+    pub created_at: DateTime<Utc>,
+}
+
+#[derive(Debug, Clone)]
+pub struct Boost {
+    pub id: BoostId,
+    pub user_id: UserId,
+    pub thought_id: ThoughtId,
+    pub ap_id: Option<String>,
+    pub created_at: DateTime<Utc>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum FollowState { Pending, Accepted, Rejected }
+impl FollowState {
+    pub fn from_str(s: &str) -> Self {
+        match s { "pending" => Self::Pending, "rejected" => Self::Rejected, _ => Self::Accepted }
+    }
+    pub fn as_str(&self) -> &str {
+        match self { Self::Pending => "pending", Self::Accepted => "accepted", Self::Rejected => "rejected" }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct Follow {
+    pub follower_id: UserId,
+    pub following_id: UserId,
+    pub state: FollowState,
+    pub ap_id: Option<String>,
+    pub created_at: DateTime<Utc>,
+}
+
+#[derive(Debug, Clone)]
+pub struct Block {
+    pub blocker_id: UserId,
+    pub blocked_id: UserId,
+    pub created_at: DateTime<Utc>,
+}
diff --git a/crates/domain/src/models/tag.rs b/crates/domain/src/models/tag.rs
new file mode 100644
index 0000000..9c78590
--- /dev/null
+++ b/crates/domain/src/models/tag.rs
@@ -0,0 +1,2 @@
+#[derive(Debug, Clone)]
+pub struct Tag { pub id: i32, pub name: String }
diff --git a/crates/domain/src/models/thought.rs b/crates/domain/src/models/thought.rs
new file mode 100644
index 0000000..56a1869
--- /dev/null
+++ b/crates/domain/src/models/thought.rs
@@ -0,0 +1,45 @@
+use chrono::{DateTime, Utc};
+use crate::value_objects::{ThoughtId, UserId, Content};
+
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub enum Visibility {
+    Public, Followers, Unlisted, Direct,
+}
+impl Visibility {
+    pub fn from_str(s: &str) -> Self {
+        match s { "followers" => Self::Followers, "unlisted" => Self::Unlisted, "direct" => Self::Direct, _ => Self::Public }
+    }
+    pub fn as_str(&self) -> &str {
+        match self { Self::Public => "public", Self::Followers => "followers", Self::Unlisted => "unlisted", Self::Direct => "direct" }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct Thought {
+    pub id: ThoughtId,
+    pub user_id: UserId,
+    pub content: Content,
+    pub in_reply_to_id: Option<ThoughtId>,
+    pub in_reply_to_url: Option<String>,
+    pub ap_id: Option<String>,
+    pub visibility: Visibility,
+    pub content_warning: Option<String>,
+    pub sensitive: bool,
+    pub local: bool,
+    pub created_at: DateTime<Utc>,
+    pub updated_at: Option<DateTime<Utc>>,
+}
+
+impl Thought {
+    pub fn new_local(
+        id: ThoughtId, user_id: UserId, content: Content,
+        in_reply_to_id: Option<ThoughtId>, visibility: Visibility,
+        content_warning: Option<String>, sensitive: bool,
+    ) -> Self {
+        Self {
+            id, user_id, content, in_reply_to_id, in_reply_to_url: None, ap_id: None,
+            visibility, content_warning, sensitive, local: true,
+            created_at: Utc::now(), updated_at: None,
+        }
+    }
+}
diff --git a/crates/domain/src/models/top_friend.rs b/crates/domain/src/models/top_friend.rs
new file mode 100644
index 0000000..d0d3279
--- /dev/null
+++ b/crates/domain/src/models/top_friend.rs
@@ -0,0 +1,4 @@
+use crate::value_objects::UserId;
+
+#[derive(Debug, Clone)]
+pub struct TopFriend { pub user_id: UserId, pub friend_id: UserId, pub position: i16 }
diff --git a/crates/domain/src/models/user.rs b/crates/domain/src/models/user.rs
new file mode 100644
index 0000000..0b19f98
--- /dev/null
+++ b/crates/domain/src/models/user.rs
@@ -0,0 +1,35 @@
+use chrono::{DateTime, Utc};
+use crate::value_objects::{UserId, Username, Email, PasswordHash};
+
+#[derive(Debug, Clone)]
+pub struct User {
+    pub id: UserId,
+    pub username: Username,
+    pub email: Email,
+    pub password_hash: PasswordHash,
+    pub display_name: Option<String>,
+    pub bio: Option<String>,
+    pub avatar_url: Option<String>,
+    pub header_url: Option<String>,
+    pub custom_css: Option<String>,
+    pub local: bool,
+    pub ap_id: Option<String>,
+    pub inbox_url: Option<String>,
+    pub public_key: Option<String>,
+    pub private_key: Option<String>,
+    pub created_at: DateTime<Utc>,
+    pub updated_at: DateTime<Utc>,
+}
+
+impl User {
+    pub fn new_local(id: UserId, username: Username, email: Email, password_hash: PasswordHash) -> Self {
+        let now = Utc::now();
+        Self {
+            id, username, email, password_hash,
+            display_name: None, bio: None, avatar_url: None, header_url: None,
+            custom_css: None, local: true, ap_id: None, inbox_url: None,
+            public_key: None, private_key: None,
+            created_at: now, updated_at: now,
+        }
+    }
+}
diff --git a/crates/domain/src/ports.rs b/crates/domain/src/ports.rs
new file mode 100644
index 0000000..729d0f1
--- /dev/null
+++ b/crates/domain/src/ports.rs
@@ -0,0 +1 @@
+// filled in Task 4
diff --git a/crates/domain/src/testing.rs b/crates/domain/src/testing.rs
new file mode 100644
index 0000000..729d0f1
--- /dev/null
+++ b/crates/domain/src/testing.rs
@@ -0,0 +1 @@
+// filled in Task 4