The pion plays an outstanding role in nuclear physics as a generator of the nuclear force and as an important part of the nuclear many-body problem. At the same time, pion beams are used as probes to explore the nucleus and its interactions. As pion physics has matured over the years, a rich and diverse variety of phenomena has been uncovered. The aim of this book is to examine the underlying physical picture behind these phenomena in a systematic and coherent way. The authors emphasize the interplay between physical concepts and experimental facts more than the formal tools, which are presented in a series of appendices. The book is directed towards advanced students as well as research workers.