This book is a comprehensive work on model-based diagnosis of a broad class of discrete-event systems, called active systems. The diagnostic approach, developed over several years by the authors, is surveyed along with introductory elements and related contributions. The area is new and the presented material is not collected in any other outlet. Each subject is dealt with both informally and formally, so that, based on a variety of examples, the reader can understand the basic ideas at an intuitive level first, while possibly delving into formal details afterwards. Based on an insight into an advanced technology, researchers and system engineers are likely to draw inspiration to cope with conceptual and practical aspects of the complex task of diagnosis. Audience: This volume will be of interest to researchers in the field of model-based diagnosis, both in academic and industrial contexts, and to students of artificial intelligence, knowledge engineering and algorithms and data structures, and to system engineers involved in the monitoring and diagnosis of industrial apparatus.