As part of the formative assessment for the Trial of Activity for Adolescent Girls (TAAG), a multicenter study to reduce the decline of physical activity in adolescent girls, girls and boys with diverse ethnicity from six states participated in focus groups and semistructured interviews. Data from 13 girls' focus groups (N = 100), 11 boys' focus groups (N = 77), and 80 semistructured interviews with girls are examined to identify perceptions of girls' physical activity behaviors to help develop TAAG interventions. Both girls and boys talk about physically active girls as being "tomboys" or "too aggressive." Girls are more likely to characterize active girls as "in shape," whereas boys say they are "too athletic." Girls report boys to be influential barriers and motivators in shaping their beliefs about physical activity. Given the strong influence of peers, developing successful interventions for girls should include verbal persuasion, modeling, and social support from both girls and boys. (Contains 1 table and 2 figures.)