The reliability of software is becoming increasingly important to a large range of industries that rely on complex computer systems and machinery with computer control. The reliability of a system depends on both the hardware and the software that comprise the system. Although faults in design can continue to give problems, the issues and the techniques for meeting severe reliability requirements in hardware have been understood for some time. In the case of software both the techniques and a positive attitude of software developers to the achievement of reliability are much less well established. They are particularly crucial in the development of software dependent safety-critical systems.