Many have argued that industrial societies are becoming more technology-dependent and are thus more vulnerable to technology failures. Despite the pervasiveness of computer technology, little is known about computer failures, except perhaps that they are all too common. This article analyses the sources of computer unreliability and reviews the extent and cost of unreliable computers. Unlike previous writers, the authors argue that digital computers are inherently unreliable for two reasons: first, they are prone to total rather than partial failure; and second, their enormous complexity means that they can never be thoroughly tested before use. The authors then describe various institutional attempts to improve reliability and possible solutions proposed by computer scientists, but they conclude that as yet none is adequate. Accordingly, they recommend that computers should not be used in life-critical applications.