The capacity to understand and reason about the unobservable mental states (e.g., thoughts, desires, and beliefs) of oneself and others, known as theory of mind (ToM), is central to human social cognition. Multidisciplinary interest in ToM stems from its potentially unique human nature, the role it plays in our ability to engage in complex social interactions, and its impairment in psychiatric and developmental disorders, such as autism. Through more than 30 years of research, we have learned a great deal about how and when children come to reason about others in terms of their mental states. This essay reviews foundational research on the development of ToM reasoning during childhood; outlines cutting-edge findings on the infant origins and neural correlates of ToM; and finally discusses key issues for future research, including reconciling infant competence with evidence of protracted conceptual development in early childhood, expanding our neuroscientific understanding of ToM and its development, and shedding light on the use and individual variability of ToM in everyday life. Pursuing these goals will address important theoretical questions and provide critical new insight into the origins, development, neural basis, and social and behavioral consequences of ToM.