This book is about complexity in Information Systems (IS). The subject is addressed from both conceptual and applied perspectives. Concepts are drawn from information theory, industrial design and software engineering. Its content capitalizes on experiences gathered by the authors during various contracting jobs involving software architecture, modeling and IS architecture that were conducted for large organizations in the banking and industry sectors, as well as in the public sector. The authors develop the point of view according to which mastering complexity involves two essential steps: first, one should develop a clear understanding of the real nature of complexity within the IS; second, one should identify the primary causes which contribute to its uncontrolled growth and organize these into a logical framework, in order to define efficient countermeasures. Both technical and psychological causes of complexity are to be considered. Two themes make up the main thread of the book: complexity and value. Both themes are quite common when considered separately, but their interplay remains a largely unexplored topic. The analysis of this interplay is one of the sources of originality of this book.