Physical activity declines with increasing age through childhood and adolescence, and a large percentage of adolescents in the U.S. do not meet current physical activity guidelines. The transition from elementary school to middle school is a critical developmental stage in the lives of young people, and it is a period during which physical activity levels decline dramatically in many children. Many previous studies have examined the correlates of physical activity behavior in children and adolescents. However, no previous studies have prospectively and comprehensively studied the mechanisms that underlie change in physical activity behavior over time in young people. Likewise, none have used an objective measure of physical activity and multi-level analytic methods. Therefore, the aims of the proposed study are to examine the direct and indirect influences of personal, home/family, neighborhood, school and community factors on changes in physical activity in African American and white boys and girls as they transition from elementary school to middle school. The potential moderating effects of gender, race, socioeconomic status, and neighborhood environment on factors influencing changes in physical activity also will be examined. The study will involve 750 5th grade students and their parents, who will participate in a measurement protocol at 5th, 6th and 7th grades. Measures will include physical activity measured by accelerometry and self-report and a wide range of independent variables measured at three levels — child/family/home/street, elementary school/neighborhood, and middle school/community. Data will be provided by children and parents during annual home visits during which questionnaires will be completed and anthropometric measures will be administered. School data will be acquired through interviews with school personnel, and information on physical environmental factors will be measured by windshield surveys of street segments, application of a detailed Geographic Information Systems database, audits of physical activity places, and self-reported perceptions of the environment. Multi-level structural equation modeling procedures will be used to identify factors that are directly or indirectly associated with change in physical activity across the three grade levels. This study will address a critical public health challenge by markedly expanding our knowledge of the factors that influence development of physical activity behavior in youth. This knowledge is needed to inform future public health interventions aimed at increasing physical activity and preventing obesity in children and adolescents.