This book is aimed at anyone who wants to understand the use of quantum theory for the description of physical processes. It is a graduate level text, ideal for independent study, and includes numerous figures, exercises, bibliographical references, and even some computer programs. The first chapters introduce formal tools; the mathematics are precise, but not excessively abstract. The physical interpretation too is rigorous. It makes no use of the uncertainty principle of other ill-defined notions. The central part of the book is devoted to Bell's theorem and to the Kochen-Specker theorem. It is here that quantum phenomena depart most radically from classical physics. There has recently been considerable progress on these issues, and current developments have been included. The final chapters discuss further topics of current research: spacetime symmetries, quantum thermodynamics and information theory, semiclassical methods, irreversibility, quantum chaos, and especially the measuring process. In particular, it is shown how modern techniques allow the extraction of more information from a physical system than traditional measurement methods. For physicists, mathematicians and philosophers of science with an interest in the applications and foundations of quantum theory. The volume is suitable as a supplementary graduate textbook.