This study deals with the development of, and the current discussion about, the interpretation of quantum mechanics. The following topics are discussed: 1. The Copenhagen Interpretation; 2. Formal Problems of Quantum Mechanics; 3. Process of Measurement and the Equation of Motion; 4. Macroscopic Level of Description; 5. Search for Hidden Variables; 6. The Notion of 'Reality' and the Epistemology of Quantum Mechanics; and 7. Quantum Mechanics and the Explanation of Life. The Bohr-Einstein dialogue on the validity of the quan­ tum mechanical description of physical reality lasted over two decades. Since the early nineteen-fifties, Eugene Wigner has provided much of the point and counterpoint of the continuing discussion on the interpretation and epistemolo­ gy of quantum mechanics. We have explored Wigner's views in some detail against the background of historical development and current debate. Professor Eugene Wigner has sustained me over many years in my work on the conceptual development of modern physics by his kindness and encouragement. This study owes its existence to his direct inspiration, and to his suggestion to me in April 1971 that it would be of interest to write an account of the interpretation of quantum me­ chanics and the current discussion about it. XII PREFACE This study was completed in September 1972. Signifi­ cant new developments have occurred since then in the dis­ cussion of questions related to the epistemology of quan­ tum mechanics.