After creating a window, the creation function returns a window handle that uniquely identifies the window. An application can use this handle to direct the action of functions to the window. Window handles have the data type HWND; applications must use this data type when declaring variables that hold window handles.

There are special constants that an application can use instead of a window handle in certain functions. For example, an application can use HWND_DESKTOP in the WinCreateWindow function to specify the desktop window as the new window's parent. Similarly, HWND_OBJECT represents the desktop-object window. HWND_TOP and HWND_BOTTOM represent the top and bottom positions relative to the z-order position of a window.

Although the NULL constant is not a window handle, an application can use it in some functions to specify that no window is affected. For example, an application can use NULL in the WinCreateWindow function to create a window that has no owner window. Some functions might return NULL, indicating that the given action applies to no window.