This Element describes the development of a Theory of Mind, or mentalizing, in infancy and early childhood. Theory of Mind is a key social cognitive ability that permits children to predict and explain human behaviors by attributing mental states to other people. Understanding mental states gradually progresses from basic desires to false beliefs. The Element reviews the proximal and distal cognitive and social determinants that facilitate early Theory of Mind development. Discoveries in neuroscience contribute to understanding the ontogeny of Theory of Mind. This Element presents an overview of the main theoretical accounts of Theory of Mind development and offers suggestions for future research.