Everyday understanding of the social world relies, at least in part, on having a theory of mind-an understanding of how mental states such as beliefs, desires, and intentions cause human behavior. After introducing the core conceptual distinctions to frame discussion of the issues, we review a wide range of literature pertaining to the development of a range of theory of mind understandings; the cognitive, neurobiological, and experiential factors that contribute to those developments; and how the development of theory of mind affects a variety of social competencies, in typical and atypical development. Finally, we discuss various theories of theory of mind development and broader importance of theory of mind research for the field.