This text describes test and diagnosis at the system level, defined as any aggregation of related elements that together form an entity of sufficient complexity for which it is impractical to treat all of the elements at the lowest level of detail. The ideas presented emphasize that it is possible to diagnose complex systems efficiently. Since the notion of system is hierarchical, these ideas are applicable to all levels. The philosophy is presented in the context of a model-based approach, using the information-flow model, that focuses on the information provided by the tests rather than the functions embedded in the system. Detailed algorithms are offered for evaluating system testability, performing efficient diagnosis, verifying and validating the models, and constructing an architecture for system maintenance. Several advanced algorithms, not commonly available in existing diagnosis tools, are discussed, including reasoning with inexact or uncertain test data, breaking large problems into manageable smaller problems, diagnosing systems with time-sensitive information and time-dependent tests and learning from experience. The book is divided into three parts. The first part provides motivation for careful development of the subject and the second part provides the tools necessary for analyzing system testability and computing diagnostic strategies. The third part presents advanced topics in diagnosis. Several case studies are provided, including a single detailed case study. Smaller case studies describe experiences from actual applications of the methods discussed. The detailed case study walks the reader through a complete analysis of a system to illustrate the concepts and describe the analyses that are possible. All case studies are based upon real systems that have been modelled for the purposes of diagnosis. System Test and Diagnosis is the culmination of nearly 12 years of research into diagnosis modelling and its applications. It is designed as a primary reference for engineers and practitioners interested in system test and diagnosis.