This volume introduces the basic philosophical and conceptual questions underlying the formulation of quantum mechanics, one of the most baffling and far-reaching aspects of modern physics. Chapters by leading thinkers in this field, inspired by the profound work of the late John Bell, debate some of the deepest issues concerning the nature of physical reality, including the theory of physical measurements, how to test quantum mechanics, and how classical and quantum physics are related. This book will engage students with a background in quantum physics who wish to explore in more detail its philosophical aspects; practicing scientists not content with blindly applying the rules of quantum mechanics; and other readers interested in gaining a deeper understanding of the philosophy of physics.