diff --git a/tests/test_single_instance.py b/tests/test_single_instance.py
new file mode 100644
index 0000000..4dca977
--- /dev/null
+++ b/tests/test_single_instance.py
@@ -0,0 +1,82 @@
+import os
+import sys
+import pytest
+from unittest.mock import MagicMock, patch
+import wecker
+
+# We need to mock the PID file existence and os.kill to test the logic
+# because we are running in a test environment.
+
+@pytest.fixture
+def mock_pid_file(tmp_path):
+    """Provides a temporary PID file path and ensures it's cleaned up."""
+    pid_file = tmp_path / "wecker.pid"
+    with patch("wecker.PID_FILE", str(pid_file)):
+        yield pid_file
+
+def test_ensure_single_instance_success(mock_pid_file):
+    """Test that the script can start if no PID file exists."""
+    # Ensure file doesn't exist
+    if mock_pid_file.exists():
+        mock_pid_file.unlink()
+
+    with patch("os.path.exists", return_value=False), \
+         patch("os.getpid", return_value=1234):
+        
+        wecker.ensure_single_instance()
+        
+        # Verify it wrote the PID
+        assert mock_pid_file.exists()
+        assert mock_pid_file.read_text() == "1234"
+
+def test_ensure_single_instance_already_running(mock_pid_file):
+    """Test that the script exits if another instance is running."""
+    # 1. Create the PID file with a dummy PID
+    mock_pid_file.write_text("5678")
+    
+    # 2. Mock os.path.exists to find the file
+    # 3. Mock os.kill to succeed (meaning process 5678 is alive)
+    # Note: We DON'T mock builtins.open here, so it reads the real file we just wrote.
+    with patch("os.path.exists", return_value=True), \
+         patch("os.kill") as mock_kill:
+        
+        mock_kill.return_value = None # Success means process is alive
+    
+        with pytest.raises(SystemExit) as excinfo:
+            wecker.ensure_single_instance()
+        
+        assert excinfo.value.code == 1
+        mock_kill.assert_called_with(5678, 0)
+
+def test_ensure_single_instance_stale_pid(mock_pid_file):
+    """Test that the script continues if the PID in the file is dead."""
+    mock_pid_file.write_text("5678")
+    
+    with patch("os.path.exists", return_value=True), \
+         patch("os.kill", side_effect=ProcessLookupError), \
+         patch("os.getpid", return_value=1234):
+        
+        wecker.ensure_single_instance()
+        
+        # Should have overwritten with new PID
+        assert mock_pid_file.read_text() == "1234"
+
+def test_remove_pid_file_success(mock_pid_file):
+    """Test that remove_pid_file removes the correct PID file."""
+    mock_pid_file.write_text("1234")
+    
+    with patch("os.path.exists", return_value=True), \
+         patch("os.getpid", return_value=1234):
+        
+        wecker.remove_pid_file()
+        assert not mock_pid_file.exists()
+
+def test_remove_pid_file_wrong_pid(mock_pid_file):
+    """Test that remove_pid_file does NOT remove if PID doesn't match."""
+    mock_pid_file.write_text("5678")
+    
+    with patch("os.path.exists", return_value=True), \
+         patch("os.getpid", return_value=1234):
+        
+        wecker.remove_pid_file()
+        assert mock_pid_file.exists()