Adolescent physical inactivity is a major public health concern. Insufficient physical activity (PA) contributes to adolescent overweight, obesity, type 2 diabetes and other morbid conditions that have escalated in recent years. These issues are particularly critical among urban minority youth, who suffer disproportionately from obesity and type 2 diabetes and have fewer resources and opportunities to engage in PA. The proposed exploratory and developmental research, responding to RFA #CA-04-009, "Mechanisms of Physical Activity Behavior Change," identifies a novel hypothesized mechanism for increasing adolescent afterschool PA: motivating and training them to identify and engage in activities that are compatible with their environmental, physiological, and psychosocial contexts and needs. This R21 proposal is to develop and pilot-test the intervention and measures for a subsequent experimental trial. Through focus groups and in-depth interviews, multiethnic 11-14 year olds in New York City will play a major role in designing visually appealing, gender-specific magazine-style materials featuring photographs and profiles of diverse types of adolescents and the PA they enjoy doing. Pediatrician approved PA practices will be included for youth with concerns such as asthma or overweight, as will creative strategies for negotiating environmental constraints and social preoccupations. The community-based intervention, to be delivered in afterschool programs, will use these print materials in 8-12 sessions of interactive workshops and individual activities to build youths' skills in: 1) self-assessing their environmental, physiological, and psychosocial contexts and preferences, 2) describing the environmental, physiological, and psychosocial "profiles" of a wide variety of physical activities, 3) identifying PAs that that are compatible with their own contexts and preferences, 4) participating in those PAs and evaluating their experiences, 5) refining and updating their PA repertoire by iteratively engaging in the compatibility-assessment process, and 6) maintaining and reinforcing their PA behaviors. The proposed research also includes instrument development and survey piloting with 300 multiethnic young adolescents, to accurately measure the environmental, physiological, and psychosocial mediators targeted by the intervention. The proposed research is expected to lead to a randomized experiment testing the efficacy of intervening on this novel mechanism for PA behavior change in youth.