1. A method of debugging an application program running under an operating system on a computer, the operating system being of a type which allows registration of callback functions with an operating system exception handler, the exception handler calling said callback functions in response to an exception generated by the application program, the method comprising the following steps: starting a monitor program on the computer, the monitor program including a monitor callback function; registering the monitor callback function with the operating system exception handler; calling the monitor callback function from the operating system exception handler in response to an exception generated by an application program; in response to calling the monitor callback function, starting a debugging program to debug the application program which generated the exception.