Computer-based systems are now essential to everyday life. They involve both tech- cal (hardware/software) components and human beings as active participants. Wh- ever we ?y aboard an aircraft or withdraw money from a cash point, a combination of humans, machines and software is supporting the delivery of the service. These systems and many others bene't from the miniaturisation and cost reduction of the hardware which has made it possible for computers to be embedded everywhere. An equally remarkable development is the software involved: today, systems are built which were literally...