This study will examine the association between neighborhood built and social environments and childhood fitness and body size by using data for about 630,000 public school students from the 2007-2008 wave of the New York City Fitnessgram, an annual assessment of fitness and body size recently implemented in NYC.  The large sample size provides ample statistical power for analysis of interactions between built environment features and individual- and neighborhood-level characteristics. Moreover, this annual assessment offers an exceptional opportunity for developing a cohort study for later follow-up and supplemental data collection. This study will inform policy interventions to address childhood obesity by showing how modifiable built environment features are associated with fitness and body size for a vulnerable population of children and youth. If resources such as parks or recreation centers are more beneficial for some groups than for others, and/or if their benefits are contingent on social conditions such as improved neighborhood safety, the findings of this research will help policymakers use scarce public resources more efficiently and effectively to promote physical activity.  The Columbia Built Environment and Health (BEH) research group, which will conduct the research, has multiple connections to the local policy process through collaborations with the NYC Department of Health and Mental Hygiene (DOHMH) and the Department of Parks and Recreation (DOPR), and through involvement with Mayor Bloomberg’s PlaNYC 2030 sustainability planning process and with the Kellogg Foundation-funded NYC Food & Fitness Partnership. These connections will ensure that the proposed research is responsive to on-the-ground realities of city planning and health promotion in a large urban context, and that research findings can be communicated quickly to city officials.