Understanding, as Descartes, Locke and Kant all insisted, is the primary 'faculty' of the mind; yet our modern sciences have been slow to advance a clear and testable account of what it means to understand, of children's acquisition of this concept and, in particular, how children come to ascribe understanding to themselves and others. By drawing together developmental and philosophical theories, this book provides a systematic account of children's concept of understanding and places understanding at the heart of children's 'theory of mind'. Children's subjective awareness of their own minds, of what they think, depends on learning a language for ascribing mental states to themselves and others. This book will appeal to researchers in developmental psychology, cognitive science, education and philosophy who are interested in the cognitive and emotional development of children and in the more basic question of what it means to have a mind.