This book is intended for the student of computing and the practising computer professional who is concerned about the unreliability of computer systems. It is the author's aim to bring an understanding of the concept of software reliability and to impart some ideas, which should lead to the development of more reliable soft­ ware systems. The book tries to bridge the gap between theory and practice and will thus be valuable supplemental reading for a course on software engineering. During my work on real-time systems in industry I have seen many occasions where the subjects of testing, error detection and error handling have been tackled in an unsystematic, ad hoc fashion which leads to subsequent problems in the integration phase. The chapters on these subjects formed the starting point of this book and other chapters were developed in order to produce a clear and concise text covering the whole subject of software reliability, while always keeping the practical aspect in mind. This English edition is a revised version of the original German edition. The author would like to thank all his friends and colleagues for their help, suggestions and remarks on the German text. Many of the comments which have been raised have been considered in this revised English version.