Humans live not only in a physical world but also in a mental world. Theory of mind reflects the understanding that the mind is comprised of different mental states, such as intentions, desires, and beliefs. This conception of the mind is a critical achievement in human development because it directly impacts effective communication and social interaction. It allows for the understanding of others’ behaviors by inferring their mental states. The formation of a theory of mind has been a central topic in psychology, neuroscience, cognitive science, and philosophy. It impacts related processes, such as communication skills, perspective-taking ability, and social cognition. Across the life span, the understanding of the mind becomes increasingly complex. Early in development, infants and toddlers can discern the intentions of others. Later, more sophisticated reasoning about the mental states of others becomes possible. For instance, the ability to follow and understand the recursive thought that “Sam believes, that Mary said, that Jose wanted . . .” develops. Additionally, within distinctive developmental time periods, people differ in their ability to take into account mental states. Once people’s beliefs, including their misconceptions, are identified, it is possible to generate effective communication strategies designed to teach, learn, and even reduce risk-taking behaviors.