diff --git a/crates/domain/src/error.rs b/crates/domain/src/error.rs
index 134cd86..005fc5c 100644
--- a/crates/domain/src/error.rs
+++ b/crates/domain/src/error.rs
@@ -1,4 +1,8 @@
-use types::{bounded::CapSlot, id::ProcessId, permission::Permission};
+use types::{
+    bounded::CapSlot,
+    id::{ProcessId, ThreadId},
+    permission::Permission,
+};
 
 #[derive(Debug)]
 pub enum KernelError {
@@ -10,4 +14,5 @@ pub enum KernelError {
     WouldBlock,
     TooManyThreadsInProcess,
     TooManyProcesses,
+    InvalidStateTransition(ThreadId),
 }
diff --git a/crates/domain/src/thread/mod.rs b/crates/domain/src/thread/mod.rs
new file mode 100644
index 0000000..38ebc97
--- /dev/null
+++ b/crates/domain/src/thread/mod.rs
@@ -0,0 +1,143 @@
+use types::{
+    bounded::{CapSlot, Priority},
+    id::{CoreId, ProcessId, ThreadId},
+    time::TimeSlice,
+};
+
+use crate::error::KernelError;
+
+#[cfg(test)]
+#[path = "tests.rs"]
+mod tests;
+
+pub struct Thread {
+    id: ThreadId,
+    process: ProcessId,
+    state: ThreadState,
+    saved_context: SavedContext,
+    priority: Priority,
+    time_slice: TimeSlice,
+    core_affinity: Option<CoreId>,
+}
+
+pub enum ThreadState {
+    Ready,
+    Running(CoreId),
+    Blocked(BlockReason),
+    Dead,
+}
+
+pub struct SavedContext {
+    registers: [usize; 32],
+}
+
+impl SavedContext {
+    pub fn empty() -> Self {
+        Self { registers: [0; 32] }
+    }
+}
+
+pub enum BlockReason {
+    Sending(CapSlot),
+    Receiving(CapSlot),
+    WaitingForInterrupt(CapSlot),
+    WaitingAny,
+}
+
+impl Thread {
+    pub fn new(
+        id: ThreadId,
+        process: ProcessId,
+        priority: Priority,
+        time_slice: TimeSlice,
+        core_affinity: Option<CoreId>,
+    ) -> Self {
+        Self {
+            id,
+            process,
+            state: ThreadState::Ready,
+            saved_context: SavedContext::empty(),
+            priority,
+            time_slice,
+            core_affinity,
+        }
+    }
+
+    pub fn id(&self) -> ThreadId {
+        self.id
+    }
+
+    pub fn process(&self) -> ProcessId {
+        self.process
+    }
+
+    pub fn state(&self) -> &ThreadState {
+        &self.state
+    }
+
+    pub fn priority(&self) -> Priority {
+        self.priority
+    }
+
+    pub fn time_slice(&self) -> &TimeSlice {
+        &self.time_slice
+    }
+
+    pub fn time_slice_mut(&mut self) -> &mut TimeSlice {
+        &mut self.time_slice
+    }
+
+    pub fn core_affinity(&self) -> Option<CoreId> {
+        self.core_affinity
+    }
+
+    pub fn set_running(&mut self, core: CoreId) -> Result<(), KernelError> {
+        match self.state {
+            ThreadState::Ready => {
+                self.state = ThreadState::Running(core);
+                Ok(())
+            }
+            _ => Err(KernelError::InvalidStateTransition(self.id)),
+        }
+    }
+
+    pub fn block(&mut self, reason: BlockReason) -> Result<(), KernelError> {
+        match self.state {
+            ThreadState::Running(_) => {
+                self.state = ThreadState::Blocked(reason);
+                Ok(())
+            }
+            _ => Err(KernelError::InvalidStateTransition(self.id)),
+        }
+    }
+
+    pub fn unblock(&mut self) -> Result<(), KernelError> {
+        match self.state {
+            ThreadState::Blocked(_) => {
+                self.state = ThreadState::Ready;
+                Ok(())
+            }
+            _ => Err(KernelError::InvalidStateTransition(self.id)),
+        }
+    }
+
+    pub fn preempt(&mut self) -> Result<(), KernelError> {
+        match self.state {
+            ThreadState::Running(_) => {
+                self.state = ThreadState::Ready;
+                Ok(())
+            }
+            _ => Err(KernelError::InvalidStateTransition(self.id)),
+        }
+    }
+
+    pub fn exit(&mut self) -> Result<(), KernelError> {
+        match self.state {
+            ThreadState::Running(_) => {
+                self.state = ThreadState::Dead;
+                Ok(())
+            }
+            _ => Err(KernelError::InvalidStateTransition(self.id)),
+        }
+    }
+}
diff --git a/crates/domain/src/thread/tests.rs b/crates/domain/src/thread/tests.rs
new file mode 100644
index 0000000..085da61
--- /dev/null
+++ b/crates/domain/src/thread/tests.rs
@@ -0,0 +1,158 @@
+use super::*;
+use types::bounded::Priority;
+use types::id::{CoreId, ProcessId, ThreadId};
+use types::time::TimeSlice;
+
+fn thread(id: u16) -> ThreadId {
+    ThreadId::from_raw(id)
+}
+
+fn process(id: u16) -> ProcessId {
+    ProcessId::from_raw(id)
+}
+
+fn core(id: u8) -> CoreId {
+    CoreId::from_raw(id)
+}
+
+fn prio(value: u8) -> Priority {
+    Priority::new(value).unwrap()
+}
+
+fn time_slice() -> TimeSlice {
+    TimeSlice::new(10)
+}
+
+fn test_thread(id: u16, proc_id: u16) -> Thread {
+    Thread::new(thread(id), process(proc_id), prio(10), time_slice(), None)
+}
+
+// --- SavedContext tests ---
+
+#[test]
+fn saved_context_empty_creates_zeroed() {
+    let ctx = SavedContext::empty();
+    let _ = ctx;
+}
+
+// --- Thread construction tests ---
+
+#[test]
+fn new_thread_is_ready() {
+    let t = test_thread(0, 0);
+    assert!(matches!(t.state(), ThreadState::Ready));
+}
+
+#[test]
+fn accessors_return_construction_values() {
+    let t = Thread::new(thread(5), process(3), prio(20), time_slice(), Some(core(1)));
+    assert_eq!(t.id(), thread(5));
+    assert_eq!(t.process(), process(3));
+    assert_eq!(t.priority(), prio(20));
+    assert_eq!(t.core_affinity(), Some(core(1)));
+}
+
+// --- Valid state transitions ---
+
+#[test]
+fn ready_to_running() {
+    let mut t = test_thread(0, 0);
+    t.set_running(core(0)).unwrap();
+    assert!(matches!(t.state(), ThreadState::Running(c) if *c == core(0)));
+}
+
+#[test]
+fn running_to_blocked() {
+    let mut t = test_thread(0, 0);
+    t.set_running(core(0)).unwrap();
+    t.block(BlockReason::WaitingAny).unwrap();
+    assert!(matches!(t.state(), ThreadState::Blocked(BlockReason::WaitingAny)));
+}
+
+#[test]
+fn blocked_to_ready() {
+    let mut t = test_thread(0, 0);
+    t.set_running(core(0)).unwrap();
+    t.block(BlockReason::WaitingAny).unwrap();
+    t.unblock().unwrap();
+    assert!(matches!(t.state(), ThreadState::Ready));
+}
+
+#[test]
+fn running_to_ready_via_preempt() {
+    let mut t = test_thread(0, 0);
+    t.set_running(core(0)).unwrap();
+    t.preempt().unwrap();
+    assert!(matches!(t.state(), ThreadState::Ready));
+}
+
+#[test]
+fn running_to_dead() {
+    let mut t = test_thread(0, 0);
+    t.set_running(core(0)).unwrap();
+    t.exit().unwrap();
+    assert!(matches!(t.state(), ThreadState::Dead));
+}
+
+// --- Invalid state transitions ---
+
+#[test]
+fn dead_cannot_transition() {
+    let mut t = test_thread(0, 0);
+    t.set_running(core(0)).unwrap();
+    t.exit().unwrap();
+    assert!(t.set_running(core(0)).is_err());
+    assert!(t.block(BlockReason::WaitingAny).is_err());
+    assert!(t.unblock().is_err());
+    assert!(t.preempt().is_err());
+    assert!(t.exit().is_err());
+}
+
+#[test]
+fn ready_cannot_block() {
+    let mut t = test_thread(0, 0);
+    assert!(t.block(BlockReason::WaitingAny).is_err());
+}
+
+#[test]
+fn blocked_cannot_run_directly() {
+    let mut t = test_thread(0, 0);
+    t.set_running(core(0)).unwrap();
+    t.block(BlockReason::WaitingAny).unwrap();
+    assert!(t.set_running(core(0)).is_err());
+}
+
+#[test]
+fn ready_cannot_preempt() {
+    let mut t = test_thread(0, 0);
+    assert!(t.preempt().is_err());
+}
+
+#[test]
+fn ready_cannot_exit() {
+    let mut t = test_thread(0, 0);
+    assert!(t.exit().is_err());
+}
+
+#[test]
+fn blocked_cannot_preempt() {
+    let mut t = test_thread(0, 0);
+    t.set_running(core(0)).unwrap();
+    t.block(BlockReason::WaitingAny).unwrap();
+    assert!(t.preempt().is_err());
+}
+
+// --- Time slice ---
+
+#[test]
+fn time_slice_accessible() {
+    let t = test_thread(0, 0);
+    let _ = t.time_slice();
+}
+
+#[test]
+fn time_slice_mut_ticks() {
+    let mut t = test_thread(0, 0);
+    let expired = t.time_slice_mut().tick();
+    assert!(!expired);
+}