What we call "Information System" is, on the one hand, a complex structure made of computers and software, but mostly it is an organization that provides the enterprise the processes necessary to deliver value and to optimize this value, from those IT components. The way an information system works often appears complex if not obscure; hence it seems difficult to relate decisions with effects. The goal of this book is to help devise efficient analyses of the behavior of an information system. This book is organized around nine chapters which correspond to the key questions that regularly appear when any manager works "with" or "for" the IS organization: cost, value creation, reliability, competitiveness, outsourcing ... Each chapter focuses on one of these questions. It starts with a short fiction which is used as a "case study" to introduce and state the problems or situations that occur in most companies. The core of each chapter provides an analysis based on simple models, which helps the reader come up with the solution pieces that are best suited to her/his company, and, even more importantly, explain why. This book is intended for any manager who wants to understand better how information systems work. It is also written for IS managers who are sometimes at loss to explain the constraints of their trade. It will also be quite useful to college students who want to learn about information systems.