A window procedure is a function that receives and processes all input and requests for action sent to the windows. Every window class has a window procedure; every window created using that class uses that window procedure to respond to messages.

The system sends a message to the window procedure by passing the message data as arguments. The window procedure takes the appropriate action for the given message. Most window procedures check the message identifier, then use the information specified by the message parameters to carry out the request. When it has completed processing the message, the window procedure returns a message result. Each message has a particular set of possible return values. The window procedure must return the appropriate value for the processing it performed.

A window procedure cannot ignore a message. If it does not process a message, it must pass the message back to the operating system for default processing. The window procedure does this by calling the WinDefWindowProc function to carry out a default action and return the message result. Then, the window procedure must return this value as its own message result.

A window procedure commonly processes messages for several windows. It uses the window handle specified in the message to identify the appropriate window. Most window procedures process just a few types of messages and pass the others on to the operating system by calling WinDefWindowProc.