Technical Abstract: Identity theorists maintain that domain-specific self-concepts help explain the differential investment of people's time and effort in various activities. This study examined the contribution of athletic identity and three key demographic variables to physical activity and sports team participation. Students in Grades 4–5 (n=391, mean age 9.9 years, range 8–13 years, collected in 2003) and Grades 7–8 (n=948, mean age 13.6 years, range 11–15 years, collected in 2002 and 2006) completed the 40-item Athletic Identity Questionnaire, which measures self-perceptions of athletic appearance; competence; importance of physical activity and sports; and encouragement for activity from parents, teachers, and friends. Hierarchic multiple regression analyses in 2008 assessed the effects of athletic identity, race/ethnicity group, gender, and overweight status on 7-day moderate-to-vigorous physical activity (MVPA) and organized sport team participation in each age group. In children and adolescents, the global score of athletic identity was independently, positively related to MVPA (p<0.0001, p<0.0001, respectively) and team participation (p<0.0001, p<0.0001, respectively), after controlling for demographic variables. More variance in MVPA was explained in children (23%) than in adolescents (5%), in contrast to team sports (5% in children, 15% in adolescents). In the subscale analyses, positive relationships for appearance, competence, importance, and parental encouragement persisted independent of demographic factors. Results support the role of athletic self-concept in promoting physical activity and organized sport participation in children and adolescents.