We take it for granted that a person persists over time: when we make plans, we assume that we will carry them out; when we punish someone for a crime, we assume that she is the same person as the one who committed it. Metaphysical questions underlying these assumptions point towards an area of deep existential and philosophical interest. In this volume, leading metaphysicians discuss key questions about personal identity, including 'What are we?', 'How do we persist?', and 'Which conditions guarantee our identity over time?' They discuss whether personal identity is 'complex', whereby it is analyzable in terms of simpler relations such as physical or psychological features, or whether it is 'simple', namely something that cannot be analyzed in terms of more fundamental relations. Their essays offer an innovative discussion of this topic and will be of interest to a wide readership in metaphysics.