This volume introduces some of the basic philosophical and conceptual questions underlying the formulation of quantum mechanics, one of the most baffling and far-reaching aspects of modern physics. The book consists of essays by leading thinkers in this field, who have been inspired by the profound work of the late John Bell. Some of the deepest issues concerning the nature of physical reality are debated, including the theory of physical measurements, how to test quantum mechanics, and how classical and quantum physics are related. This book will be of interest to laypeople who have heard of the mysteries of quantum mechanics and want to gain a deeper understanding, students with a background in quantum physics, wishing to explore in more detail its philosophical aspects, and practising scientists who are not content with blindly applying its rules.