diff --git a/CMakeLists.txt b/CMakeLists.txt
index 599cffb..65f4f5d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -15,7 +15,7 @@ add_subdirectory(sys)
 add_custom_command(
   OUTPUT disk.img
   COMMAND sudo sh ../scripts/build_image.sh disk.img
-  DEPENDS zion
+  DEPENDS zion test
   USES_TERMINAL
 )
 
diff --git a/zion/include/zcall.h b/zion/include/zcall.h
index 67bca77..2b35c77 100644
--- a/zion/include/zcall.h
+++ b/zion/include/zcall.h
@@ -2,7 +2,7 @@
 
 #include <stdint.h>
 
-#define Z_DEBUG_PRINT 100
-
-uint64_t ZDebug(const char* message); 
+#define Z_THREAD_EXIT 0x01
+#define Z_DEBUG_PRINT 0x100
 
+uint64_t ZDebug(const char* message);
diff --git a/zion/scheduler/process.cpp b/zion/scheduler/process.cpp
index 8557715..43bd613 100644
--- a/zion/scheduler/process.cpp
+++ b/zion/scheduler/process.cpp
@@ -25,7 +25,7 @@ Process* Process::RootProcess() {
   return proc;
 }
 
-Process::Process(uint64_t elf_ptr) : id_(gNextId++) {
+Process::Process(uint64_t elf_ptr) : id_(gNextId++), state_(RUNNING) {
   cr3_ = phys_mem::AllocatePage();
   InitializePml4(cr3_);
   CreateThread(elf_ptr);
@@ -60,3 +60,15 @@ Thread* Process::GetThread(uint64_t tid) {
   panic("Bad thread access.");
   return nullptr;
 }
+
+void Process::CheckState() {
+  ThreadEntry* entry = thread_list_front_;
+
+  while (entry != nullptr) {
+    if (entry->thread->GetState() != Thread::FINISHED) {
+      return;
+    }
+    entry = entry->next;
+  }
+  state_ = FINISHED;
+}
diff --git a/zion/scheduler/process.h b/zion/scheduler/process.h
index 3ede000..1e7d38f 100644
--- a/zion/scheduler/process.h
+++ b/zion/scheduler/process.h
@@ -7,6 +7,12 @@ class Thread;
 
 class Process {
  public:
+  enum State {
+    UNSPECIFIED,
+    SETUP,
+    RUNNING,
+    FINISHED,
+  };
   // Caller takes ownership of returned process.
   static Process* RootProcess();
   Process(uint64_t elf_ptr);
@@ -17,10 +23,17 @@ class Process {
   void CreateThread(uint64_t elf_ptr);
   Thread* GetThread(uint64_t tid);
 
+  // Checks the state of all child threads and transitions to
+  // finished if all have finished.
+  void CheckState();
+
+  State GetState() { return state_; }
+
  private:
   Process(uint64_t id, uint64_t cr3) : id_(id), cr3_(cr3) {}
   uint64_t id_;
   uint64_t cr3_;
+  State state_;
 
   uint64_t next_thread_id_ = 0;
 
diff --git a/zion/scheduler/scheduler.cpp b/zion/scheduler/scheduler.cpp
index 2930d2d..76cde33 100644
--- a/zion/scheduler/scheduler.cpp
+++ b/zion/scheduler/scheduler.cpp
@@ -34,6 +34,15 @@ class ProcList {
     };
   }
 
+  void DumpProcessStates() {
+    ProcEntry* p = front_;
+    dbgln("Process States:");
+    while (p != nullptr) {
+      dbgln("%u: %u", p->proc->id(), p->proc->GetState());
+      p = p->next;
+    }
+  }
+
  private:
   struct ProcEntry {
     Process* proc;
@@ -71,16 +80,26 @@ class Scheduler {
     asm volatile("cli");
 
     if (current_thread_->next_thread_ == nullptr) {
-      dbgln("No next thread, continue");
-      return;
+      if (current_thread_->GetState() == Thread::RUNNING) {
+        dbgln("No next thread, continue");
+        return;
+      } else {
+        proc_list_.DumpProcessStates();
+        panic("FIXME: Implement Sleep");
+      }
     }
 
     Thread* prev = current_thread_;
     current_thread_ = current_thread_->next_thread_;
     prev->next_thread_ = nullptr;
-    if (prev->pid() != 0) {
+    if (prev->pid() != 0 && prev->GetState() == Thread::RUNNING) {
+      prev->SetState(Thread::RUNNABLE);
       Enqueue(prev);
     }
+    if (current_thread_->GetState() != Thread::RUNNABLE) {
+      panic("Non-runnable thread in the queue");
+    }
+    current_thread_->SetState(Thread::RUNNING);
     context_switch(prev->Rsp0Ptr(), current_thread_->Rsp0Ptr());
 
     asm volatile("sti");
diff --git a/zion/scheduler/thread.cpp b/zion/scheduler/thread.cpp
index fb4fef7..2521abc 100644
--- a/zion/scheduler/thread.cpp
+++ b/zion/scheduler/thread.cpp
@@ -47,3 +47,10 @@ void Thread::Init() {
   SetRsp0(rsp0_start_);
   jump_user_space(rip, rsp);
 }
+
+void Thread::Exit() {
+  dbgln("[%u.%u] Exiting", pid(), id_);
+  state_ = FINISHED;
+  process_->CheckState();
+  sched::Yield();
+}
diff --git a/zion/scheduler/thread.h b/zion/scheduler/thread.h
index b442dc5..06f4a52 100644
--- a/zion/scheduler/thread.h
+++ b/zion/scheduler/thread.h
@@ -7,6 +7,14 @@ class Process;
 
 class Thread {
  public:
+  enum State {
+    UNSPECIFIED,
+    CREATED,
+    RUNNING,
+    RUNNABLE,
+    BLOCKED,
+    FINISHED,
+  };
   static Thread* RootThread(Process* root_proc);
 
   explicit Thread(Process* proc, uint64_t tid, uint64_t elf_ptr);
@@ -22,6 +30,11 @@ class Thread {
   // Called the first time the thread starts up.
   void Init();
 
+  // State Management.
+  State GetState() { return state_; };
+  void SetState(State state) { state_ = state; }
+  void Exit();
+
   // FIXME: Probably make this private.
   Thread* next_thread_;
 
@@ -30,6 +43,7 @@ class Thread {
   Thread(Process* proc) : process_(proc), id_(0) {}
   Process* process_;
   uint64_t id_;
+  State state_ = RUNNABLE;
 
   uint64_t elf_ptr_;
 
diff --git a/zion/syscall/syscall.cpp b/zion/syscall/syscall.cpp
index 4b78e88..d13ede3 100644
--- a/zion/syscall/syscall.cpp
+++ b/zion/syscall/syscall.cpp
@@ -56,8 +56,12 @@ void InitSyscall() {
 extern "C" void SyscallHandler(uint64_t call_id, char* message) {
   Thread& thread = sched::CurrentThread();
   switch (call_id) {
+    case Z_THREAD_EXIT:
+      thread.Exit();
+      panic("Returned from thread exit");
+      break;
     case Z_DEBUG_PRINT:
-      dbgln("[%u.%u] %s", thread.pid(), thread.tid(), message);
+      dbgln("[%u.%u] [Debug] %s", thread.pid(), thread.tid(), message);
       break;
     default:
       panic("Unhandled syscall number: %u", call_id);