The California Child Q-set (CCQ) was used to explore the structure of personality in early adolescence and to develop scales to measure the "Big Five" dimensions: Extraversion, Agreeableness, Conscientiousness, Neuroticism, and Openness to Experience. Mothers provided Q-sorts of 350 ethnically diverse boys between 12 and 13 years old. Analyses of the construct validity of the scales provided a nomological network relating the Big Five to theoretically and socially important criterion variables, such as juvenile delinquency, Externalizing and Internalizing disorders of childhood psychopathology, school performance, IQ, SES, and race. These effects were obtained using diverse methods, including self-reports from the boys, ratings by their mothers and their teachers, and objective-test data. In addition to the Big Five, analyses also suggested 2 possibly age-specific dimensions of personality in early adolescence. Discussion is focused on the changing manifestations of personality traits throughout development.