Robert Stalnaker draws together in this volume his seminal work in metaphysics. The central theme is the role of possible worlds in articulating our various metaphysical commitments. The book begins with reflections on the general idea of a possible world, and then uses the framework of possible worlds to formulate and clarify some questions about properties and individuals, reference, thought, and experience. The essays also reflect on the nature of metaphysics, and on the relation between questions about what there is and questions about how we talk and think about what there is. Two of the fourteen essays, plus an extensive introduction that sets the papers in context and draws out the essays' common threads, are published here for the first time.