1. Apparatus for visualizing operation of a system implemented by means of processes running in at least one computer system, the apparatus comprising: monitoring means for causing at least one of the processes to send a message when the process performs an operating system call, the message indicating at least the process and the operating system call; and means for receiving the message and responding thereto by making an alteration in a graphical display of the system which includes at least an entity representing the one process, the alteration reflecting an effect on the system of the operating system call which resulted in the message being sent.