By the age of 4 or 5 children come to understand that other people have mental states such as desires, beliefs, hopes, and fears. This understanding provides a foundation for development of social knowledge and social skills. For example, realization that other people are mental beings like oneself is prerequisite for developing an understanding of why people behave as they do, and how to interact successfully with others. This project investigates an important issue that has received little attention: How do children use their understanding of mental states in social interactions? Specifically, the project focuses on children's ability to use mental state understanding in attempts at persuasion, and how this ability develops between age 4 and adulthood. One study will examine children's persuasion attempts in real-life situations through analyses of a large database of children's speech. In other experiments children will be presented with scenarios involving persuasion (e.g., a child attempting to persuade a parent to buy an animal), and asked to create or select a persuasive argument. The results should further our understanding of children's cognitive and social development, and should also have implications for educating children about people and the social world.