This book introduces the reliability technology of complex software systems. In this book, the authors first introduce the basic concepts of complex software systems, then the authors analyze the principles that may cause the failure of complex software systems, and the authors also present how to establish the proper reliability models. The authors introduce the architecture-based and data-based reliability assessment methods, model-based testing methods, and human factors related to complex software reliability. This book not only provides a complete introduction to some basic concepts and theories, but also contains many case studies. It can be used as a textbook for graduate students in reliability engineering, and it can also be used as a reference book for advanced undergraduate students of various majors who are interested in complex software systems. This book can also be used as a reference for scientific researchers involved in equipment software development and quality management.