The purpose of the multicenter randomized trial is to test the effectiveness of a multicomponent school-based and community-linked intervention in preventing the decline in physical activity levels and cardiovascular fitness in middle school girls (i.e., in grades 6-8). The interventions will provide skills-building, supportive environments, and opportunities for participation in physical activity during and outside of the school day. Phase 1 will be 24 months for protocol development and pilot work, Phase II is 44 months for two years of intervention and one year of follow-up, and Phase III is 4 months for close out of the Study Centers, and 16 months for the Coordinating Center to collaboratively analyze and report the results.