This chapter reviews theory and research on the development of children's knowledge about the mental world, focusing especially on work done during the past 15 years under the rubric of theory-of-mind development. The three principal approaches to explaining this development--theory theory, modular theory, and simulation theory--are described first. Next comes a description of infant precursors or protoforms of theory-of-mind knowledge in infancy, including a beginning awareness of the intentionality and goal-directedness of human actions. This discussion is followed by a summary of the postinfancy development of children's understanding of visual perception, attention, desires, emotions, intentions, beliefs, knowledge, pretense, and thinking. Briefly considered next are intracultural, intercultural, and interspecies differences in theory-of-mind development. The chapter then concludes with some guesses about the future of the field.