The proposed study will establish a prospective pediatric cohort to identify intervention targets based on the location, timing, barriers, and facilitators of current physical activity and sedentary behavior in a child's day. The project will be a prospective examination of 340 African American and White girls and boys aged 10 to 16 years, including 50% who are classified as severely obese. State-of-the-art technology will be used including accelerometry to quantify physical activity, magnetic resonance imaging to quantify fat accumulation, and geographic position system and ecological momentary assessment to identify environmental and socio-emotional barriers and facilitators. In further detail, this study will examine the following objectives: