diff --git a/Cargo.toml b/Cargo.toml
index 90e1744..c2e49e5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,6 +9,7 @@ license = "MIT OR Apache-2.0"
 repository = "https://github.com/Starry-OS/starry-process"
 
 [dependencies]
+bitflags = "2.6"
 kspin = "0.1"
 lazyinit = "0.2.1"
 weak-map = "0.1"
diff --git a/src/process.rs b/src/process.rs
index 3ccff29..89971e3 100644
--- a/src/process.rs
+++ b/src/process.rs
@@ -5,9 +5,10 @@ use alloc::{
 };
 use core::{
     fmt,
-    sync::atomic::{AtomicBool, Ordering},
+    sync::atomic::{AtomicU8, Ordering},
 };
 
+use bitflags::bitflags;
 use kspin::SpinNoIrq;
 use lazyinit::LazyInit;
 use weak_map::StrongMap;
@@ -21,10 +22,19 @@ pub(crate) struct ThreadGroup {
     pub(crate) group_exited: bool,
 }
 
+bitflags! {
+    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
+    pub(crate) struct ProcessState: u8 {
+        const RUNNING = 1 << 0;
+        const STOPPED = 1 << 1;
+        const ZOMBIE  = 1 << 2;
+    }
+}
+
 /// A process.
 pub struct Process {
     pid: Pid,
-    is_zombie: AtomicBool,
+    state: AtomicU8,
     pub(crate) tg: SpinNoIrq<ThreadGroup>,
 
     // TODO: child subreaper9
@@ -191,10 +201,110 @@ impl Process {
 impl Process {
     /// Returns `true` if the [`Process`] is a zombie process.
     pub fn is_zombie(&self) -> bool {
-        self.is_zombie.load(Ordering::Acquire)
+        let bits = self.state.load(Ordering::Acquire);
+        ProcessState::from_bits_truncate(bits).contains(ProcessState::ZOMBIE)
+    }
+
+    /// Returns `true` if the [`Process`] is running.
+    pub fn is_running(&self) -> bool {
+        let bits = self.state.load(Ordering::Acquire);
+        ProcessState::from_bits_truncate(bits).contains(ProcessState::RUNNING)
+    }
+
+    /// Returns `true` if the [`Process`] is stopped.
+    pub fn is_stopped(&self) -> bool {
+        let bits = self.state.load(Ordering::Acquire);
+        ProcessState::from_bits_truncate(bits).contains(ProcessState::STOPPED)
+    }
+
+    /// Change the [`Process`] from `Running` to `Stopped`.
+    ///
+    /// This method atomically transitions the process state to STOPPED using
+    /// CAS, ensuring the state is either successfully changed or already in
+    /// ZOMBIE state (in which case no change occurs).
+    ///
+    /// # Memory Ordering
+    ///
+    /// Uses `Release` ordering on success to synchronize with `Acquire` loads
+    /// in `is_stopped()`. This ensures that any writes before this
+    /// transition, such as setting `stop_signal` in the
+    /// `ProcessSignalManager`, are visible to threads that observe the
+    /// `STOPPED` state transition.
+    pub fn transition_to_stopped(&self) {
+        let _ = self.state.fetch_update(
+            Ordering::Release, // Success: synchronize with is_stopped()
+            Ordering::Relaxed, // Failure: no synchronization needed
+            |curr| {
+                let mut flags = ProcessState::from_bits_truncate(curr);
+                if flags.contains(ProcessState::ZOMBIE) || !flags.contains(ProcessState::RUNNING) {
+                    None // Already zombie or not running, don't transition
+                } else {
+                    flags.remove(ProcessState::RUNNING);
+                    flags.insert(ProcessState::STOPPED);
+                    Some(flags.bits())
+                }
+            },
+        );
+    }
+
+    /// Change the [`Process`] from `Stopped` to `Running`.
+    ///
+    /// This method atomically transitions the process state to RUNNING using
+    /// CAS. The transition succeeds if and only if the current state is
+    /// `STOPPED` and not `ZOMBIE`.
+    ///
+    /// # Memory Ordering
+    ///
+    /// Uses `Release` ordering on success to synchronize with `Acquire` loads
+    /// in `is_running()`. This ensures that any writes before this
+    /// transition, for example, setting `cont_signal` in the
+    /// `ProcessSignalManager`, are visible to threads that observe the
+    /// `RUNNING` state transition.
+    pub fn transition_to_running(&self) {
+        let _ = self.state.fetch_update(
+            Ordering::Release, // Success: synchronize with is_running()
+            Ordering::Relaxed, // Failure: no synchronization needed
+            |curr| {
+                let mut flags = ProcessState::from_bits_truncate(curr);
+                if !flags.contains(ProcessState::STOPPED) || flags.contains(ProcessState::ZOMBIE) {
+                    None // Not stopped or already zombie, don't transition
+                } else {
+                    flags.remove(ProcessState::STOPPED);
+                    flags.insert(ProcessState::RUNNING);
+                    Some(flags.bits())
+                }
+            },
+        );
+    }
+
+    /// Change the [`Process`] from any non-ZOMBIE state to `Zombie`.
+    ///
+    /// This is a terminal state transition - once a process becomes a zombie,
+    /// it cannot transition to any other state (it can only be freed via
+    /// `free()`).
+    ///
+    /// # Memory Ordering
+    ///
+    /// Uses `Release` ordering to synchronize with `Acquire` loads in
+    /// `is_zombie()`, ensuring that when a parent process's `wait()` observes
+    /// the ZOMBIE state, it also observes all writes that happened before
+    /// the transition, particularly the exit_code set by `exit_thread()`.
+    pub fn transition_to_zombie(&self) {
+        let _ = self
+            .state
+            .fetch_update(Ordering::Release, Ordering::Relaxed, |curr| {
+                let mut flags = ProcessState::from_bits_truncate(curr);
+                if flags.contains(ProcessState::ZOMBIE) {
+                    None // Already zombie
+                } else {
+                    flags.remove(ProcessState::RUNNING | ProcessState::STOPPED);
+                    flags.insert(ProcessState::ZOMBIE);
+                    Some(flags.bits())
+                }
+            });
     }
 
-    /// Terminates the [`Process`], marking it as a zombie process.
+    /// Terminates the [`Process`].
     ///
     /// Child processes are inherited by the init process or by the nearest
     /// subreaper process.
@@ -209,7 +319,6 @@ impl Process {
         }
 
         let mut children = self.children.lock(); // Acquire the lock first
-        self.is_zombie.store(true, Ordering::Release);
 
         let mut reaper_children = reaper.children.lock();
         let reaper = Arc::downgrade(reaper);
@@ -266,7 +375,7 @@ impl Process {
 
         let process = Arc::new(Process {
             pid,
-            is_zombie: AtomicBool::new(false),
+            state: AtomicU8::new(ProcessState::RUNNING.bits()),
             tg: SpinNoIrq::new(ThreadGroup::default()),
             children: SpinNoIrq::new(StrongMap::new()),
             parent: SpinNoIrq::new(parent.as_ref().map(Arc::downgrade).unwrap_or_default()),
diff --git a/tests/group.rs b/tests/group.rs
index 2e1b7ec..50f1778 100644
--- a/tests/group.rs
+++ b/tests/group.rs
@@ -59,6 +59,7 @@ fn cleanup() {
     assert!(group.upgrade().is_some());
 
     child.exit();
+    child.transition_to_zombie();
     child.free();
     drop(child);
     assert!(group.upgrade().is_none());
@@ -134,6 +135,7 @@ fn cleanup_processes() {
     let group = parent.create_group().unwrap();
 
     parent.exit();
+    parent.transition_to_zombie();
     parent.free();
     drop(parent);
 
diff --git a/tests/process.rs b/tests/process.rs
index b91eb0d..3e782b5 100644
--- a/tests/process.rs
+++ b/tests/process.rs
@@ -1,4 +1,8 @@
-use std::sync::Arc;
+use std::{
+    sync::{Arc, Barrier},
+    thread,
+    time::Duration,
+};
 
 use starry_process::init_proc;
 
@@ -18,6 +22,7 @@ fn exit() {
     let parent = init_proc();
     let child = parent.new_child();
     child.exit();
+    child.transition_to_zombie();
     assert!(child.is_zombie());
     assert!(parent.children().iter().any(|c| Arc::ptr_eq(c, &child)));
 }
@@ -33,6 +38,7 @@ fn free() {
     let parent = init_proc().new_child();
     let child = parent.new_child();
     child.exit();
+    child.transition_to_zombie();
     child.free();
     assert!(parent.children().is_empty());
 }
@@ -70,4 +76,129 @@ fn thread_exit() {
     let last2 = child.exit_thread(102, 3);
     assert!(last2);
     assert_eq!(child.exit_code(), 7);
-}
\ No newline at end of file
+}
+
+#[test]
+fn state_lifecycle() {
+    let process = init_proc().new_child();
+
+    // Initial state should be RUNNING
+    assert!(process.is_running());
+    assert!(!process.is_stopped());
+    assert!(!process.is_zombie());
+
+    // RUNNING -> STOPPED
+    process.transition_to_stopped();
+    assert!(!process.is_running());
+    assert!(process.is_stopped());
+    assert!(!process.is_zombie());
+
+    // STOPPED -> RUNNING
+    process.transition_to_running();
+    assert!(process.is_running());
+    assert!(!process.is_stopped());
+    assert!(!process.is_zombie());
+
+    // RUNNING -> ZOMBIE
+    process.transition_to_zombie();
+    assert!(!process.is_running());
+    assert!(!process.is_stopped());
+    assert!(process.is_zombie());
+}
+
+#[test]
+fn invalid_transitions() {
+    let process = init_proc().new_child();
+
+    // STOPPED -> STOPPED (idempotent)
+    process.transition_to_stopped();
+    assert!(process.is_stopped());
+    process.transition_to_stopped();
+    assert!(process.is_stopped());
+
+    // STOPPED -> ZOMBIE
+    process.transition_to_zombie();
+    assert!(process.is_zombie());
+
+    // ZOMBIE -> RUNNING (Invalid)
+    process.transition_to_running();
+    assert!(process.is_zombie());
+    assert!(!process.is_running());
+
+    // ZOMBIE -> STOPPED (Invalid)
+    process.transition_to_stopped();
+    assert!(process.is_zombie());
+    assert!(!process.is_stopped());
+}
+
+#[test]
+fn concurrent_transitions() {
+    let process = init_proc().new_child();
+    let barrier = Arc::new(Barrier::new(4));
+
+    // Strategy:
+    // Spawn 4 threads to change the state concurrently.
+    // All of them start at the same time after all 4 barriers are reached,
+    // simulating multiple kernel threads accessing and changing the process state
+    // simultaneously.
+    let mut handles = vec![];
+
+    // Thread 1: Tries to stop
+    let p1 = process.clone();
+    let b1 = barrier.clone();
+    handles.push(thread::spawn(move || {
+        b1.wait();
+        for _ in 0..1000 {
+            p1.transition_to_stopped();
+            thread::yield_now();
+        }
+    }));
+
+    // Thread 2: Tries to continue
+    let p2 = process.clone();
+    let b2 = barrier.clone();
+    handles.push(thread::spawn(move || {
+        b2.wait();
+        for _ in 0..1000 {
+            p2.transition_to_running();
+            thread::yield_now();
+        }
+    }));
+
+    // Thread 3: Validates terminal state consistency
+    // validate the terminal state property:
+    // once a process is a zombie, it should always be a zombie.
+    let p3 = process.clone();
+    let b3 = barrier.clone();
+    handles.push(thread::spawn(move || {
+        b3.wait();
+        let mut observed_zombie = false;
+        for _ in 0..1000 {
+            if p3.is_zombie() {
+                observed_zombie = true;
+            } else if observed_zombie {
+                // If we previously saw zombie, we should never see non-zombie
+                panic!("Process transitioned from zombie to non-zombie state!");
+            }
+            thread::yield_now();
+        }
+    }));
+
+    // Thread 4: The killer
+    let p4 = process.clone();
+    let b4 = barrier.clone();
+    handles.push(thread::spawn(move || {
+        b4.wait();
+        thread::sleep(Duration::from_millis(50));
+        p4.transition_to_zombie();
+    }));
+
+    for h in handles {
+        h.join().unwrap();
+    }
+
+    // Must be zombie at the end
+    assert!(process.is_zombie());
+    assert!(!process.is_running());
+    assert!(!process.is_stopped());
+}
diff --git a/tests/session.rs b/tests/session.rs
index 71269ee..f268bff 100644
--- a/tests/session.rs
+++ b/tests/session.rs
@@ -1,5 +1,4 @@
-use std::sync::Arc;
-use std::any::Any;
+use std::{any::Any, sync::Arc};
 
 use starry_process::init_proc;
 
@@ -59,6 +58,7 @@ fn cleanup() {
 
     assert!(session.upgrade().is_some());
     child.exit();
+    child.transition_to_zombie();
     child.free();
     drop(child);
     assert!(session.upgrade().is_none());
@@ -102,6 +102,7 @@ fn cleanup_groups() {
     let (session, _) = child.create_session().unwrap();
 
     child.exit();
+    child.transition_to_zombie();
     child.free();
     drop(child);
 
@@ -129,4 +130,4 @@ fn terminal_set_unset() {
 
     assert!(session.unset_terminal(&term));
     assert!(session.terminal().is_none());
-}
\ No newline at end of file
+}