diff --git a/test/TestHeapq.fs b/test/TestHeapq.fs
index d0dd22d..9ac7f97 100644
--- a/test/TestHeapq.fs
+++ b/test/TestHeapq.fs
@@ -37,3 +37,23 @@ let ``test nlargest works`` () =
 let ``test nsmallest works`` () =
     let result = heapq.nsmallest (3, [ 1; 5; 2; 8; 3; 7 ])
     result |> equal (ResizeArray [ 1; 2; 3 ])
+
+[<Fact>]
+let ``test heapreplace pops smallest and pushes new item`` () =
+    let heap = ResizeArray [ 1; 3; 5; 7 ]
+    heapq.heapify heap
+    // Replace root (1) with 4; returns old root (1)
+    heapq.heapreplace (heap, 4) |> equal 1
+    // Smallest is now 3
+    heapq.heappop heap |> equal 3
+
+[<Fact>]
+let ``test heapreplace with larger item maintains heap`` () =
+    let heap = ResizeArray [ 2; 6; 10 ]
+    heapq.heapify heap
+    // Replace root (2) with 8; returns old root (2)
+    heapq.heapreplace (heap, 8) |> equal 2
+    // Remaining heap elements sorted: 6, 8, 10
+    heapq.heappop heap |> equal 6
+    heapq.heappop heap |> equal 8
+    heapq.heappop heap |> equal 10
diff --git a/test/TestThreading.fs b/test/TestThreading.fs
index 80d14ca..33fad23 100644
--- a/test/TestThreading.fs
+++ b/test/TestThreading.fs
@@ -18,3 +18,128 @@ let ``test local creates thread-local storage`` () =
     let local = threading.local ()
     setattr local "value" 42
     getattr local "value" 0 |> equal 42
+
+// ============================================================================
+// Lock tests
+// ============================================================================
+
+[<Fact>]
+let ``test Lock acquire and release work`` () =
+    let lock = Lock()
+    let acquired = lock.acquire ()
+    acquired |> equal true
+    lock.release ()
+
+[<Fact>]
+let ``test Lock locked reflects state`` () =
+    let lock = Lock()
+    lock.locked () |> equal false
+    lock.acquire () |> ignore
+    lock.locked () |> equal true
+    lock.release ()
+    lock.locked () |> equal false
+
+[<Fact>]
+let ``test Lock acquire non-blocking fails when locked`` () =
+    let lock = Lock()
+    lock.acquire () |> ignore
+    // Non-blocking acquire should fail since lock is already held
+    let second = lock.acquire (blocking = false)
+    second |> equal false
+    lock.release ()
+
+// ============================================================================
+// RLock tests
+// ============================================================================
+
+[<Fact>]
+let ``test RLock acquire and release work`` () =
+    let rlock = RLock()
+    let acquired = rlock.acquire ()
+    acquired |> equal true
+    rlock.release ()
+
+[<Fact>]
+let ``test RLock is reentrant`` () =
+    let rlock = RLock()
+    // Same thread can acquire multiple times
+    rlock.acquire () |> ignore
+    let second = rlock.acquire ()
+    second |> equal true
+    rlock.release ()
+    rlock.release ()
+
+// ============================================================================
+// Event tests
+// ============================================================================
+
+[<Fact>]
+let ``test Event is_set starts false`` () =
+    let ev = Event()
+    ev.is_set () |> equal false
+
+[<Fact>]
+let ``test Event set and clear work`` () =
+    let ev = Event()
+    ev.set ()
+    ev.is_set () |> equal true
+    ev.clear ()
+    ev.is_set () |> equal false
+
+[<Fact>]
+let ``test Event wait returns true when already set`` () =
+    let ev = Event()
+    ev.set ()
+    let result = ev.wait ()
+    result |> equal true
+
+[<Fact>]
+let ``test Event wait with timeout returns false when not set`` () =
+    let ev = Event()
+    // Timeout of 0 seconds — event not set, should return false immediately
+    let result = ev.wait (timeout = 0.0)
+    result |> equal false
+
+// ============================================================================
+// Thread tests
+// ============================================================================
+
+[<Fact>]
+let ``test Thread runs target function`` () =
+    let results = ResizeArray<int>()
+    let t = Thread(target = fun () -> results.Add 42)
+    t.start ()
+    t.join ()
+    results.Count |> equal 1
+    results.[0] |> equal 42
+
+[<Fact>]
+let ``test Thread is_alive reflects state`` () =
+    let ev = Event()
+    let t = Thread(target = fun () -> ev.wait () |> ignore)
+    t.is_alive () |> equal false
+    t.start ()
+    t.is_alive () |> equal true
+    ev.set ()
+    t.join ()
+    t.is_alive () |> equal false
+
+[<Fact>]
+let ``test Thread name can be set`` () =
+    let t = Thread(target = (fun () -> ()), name = "worker")
+    t.name |> equal "worker"
+
+[<Fact>]
+let ``test main_thread returns a Thread`` () =
+    let mt = threading.main_thread ()
+    mt.is_alive () |> equal true
+
+[<Fact>]
+let ``test current_thread returns a Thread`` () =
+    let ct = threading.current_thread ()
+    ct.is_alive () |> equal true
+
+[<Fact>]
+let ``test enumerate returns list with at least one thread`` () =
+    let threads = threading.enumerate ()
+    (threads.Length >= 1) |> equal true