Modified Project Summary/Abstract Section Categorization is a fundamental aspect of human cognition that allows for efficient learning, generalization, and communication. Throughout development, humans categorize the objects, people, and events around them. The proposed work examines social categorization with a focus on the adolescent period. Adolescence is a time of notable changes in cognitive and social development, brain maturation, and well-being. Using a pair of longitudinal studies, including an ongoing decade-long study and a month-long daily ecological momentary assessment study, this grant will document first and third-party social categorization changes over time in a group of adolescents. This work also investigates the role of social environmental input on adolescent social categorization by examining links between adolescents’ categorization and their parents’ categorization. Finally, in this work, we examine the links between first- and third-party social categorization and well-being. In studies including more than 2,000 research participants, this work will advance theory and practice about topics ranging from categorization and adolescent social development to health and well-being.