Psychological, biological, social, and physical environmental variables were examined for their association with physical activity of young people. A national sample of 1,504 parents and children in Grades 4-12 were interviewed by telephone. Twenty-two potential determinants were assessed along with an 11-item child physical activity index (alpha = .76). Hierarchical multiple regressions were conducted separately for 6 age-sex subgroups. Percentage of variance explained ranged from 18% for boys in Grades 4-6 to 59% for girls in Grades 10-12. Three variables had strong and consistent associations with the child physical activity index that generalized across subgroups: use of afternoon time for sports and physical activity, enjoyment of physical education, and family support for physical activity. These 3 variables should be targeted for change to promote physical activity in all groups of young people.