This review begins with a brief history from Piagetian perspective-taking development, through metacognitive development, and into the past and present field of theory-of-mind development. This field has included research on what infants and children know about a variety of mental states, on possible causes and consequences of mentalistic knowledge, and on similarities and differences in this knowledge across individuals, cultures, and primate species. The article concludes with some speculations about the future of the field.