Jeffrey Barrett presents the most comprehensive study yet of a problem that has puzzled physicists and philosophers since the 1930s. The standard theory of quantum mechanics is one of the most successful physical theories ever, predicting the behavior of the basic constituents of all physical things; no other theory has ever made such accurate empirical predictions. However, if one tries to understand the theory as a complete and accurate framework for the description of behavior of all physical interactions, it becomes evident that the theory is ambiguous, even logically inconsistent. To deal with this dilemma, in the 1950s, Hugh Everett III initiated the quantum measurement problem. Barrett gives a careful and challenging examination and evaluation of Everett's work and of those who have followed him. Barrett's informal approach and engaging narrative make this book accessible and illuminating for philosophers, physicists, and anyone interested in the interpretation of quantum mechanics.