BackgroundYouth risk of obesity is high during the summer months. Summer day camps can be ideal settings for preventing obesity through reducing youth summer sedentary behaviors. However, with limited research on camp settings, the mechanisms by which these programs promote children’s physical activity (PA) remains largely unknown. The current study was designed to take a first step in addressing this gap in research through systematic observations of 4 summer day camps.MethodsSystematic observations of 4 summer day camps was conducted using the System for Observing Play and Leisure Activity in Youth (SOPLAY) and a social-motivational climate supplemental observation tool founded on Self-Determination Theory and previous research developed by the authors. Teams of two coders observed daily activities for four days across two-week periods at each camp. On 15 minute intervals throughout each day, camps were assessed on level of youth PA (e.g., sedentary, moderate, vigorous), five physical features (e.g., equipment), eight staff interactions (e.g., encourage PA), and six social climate components (e.g., inclusive game).ResultsAcross the sample, highly engaging games [F(1,329) = 17.68, p < .001], positive peer interactions [F(1,329) = 8.43, p < .01], and bullying [F(1,329) = 9.39, p < .01] were significantly related to higher PA participation rates, and clarity of rules [F(1,329) = 11.12, p < .001] was related to fewer youth participating in PA. Separate analyses for males and females indicated some sex differences with highly engaging games [F(1,329) = 23.10, p < .001] and bullying [F(1,329) = 10.00, p < .01] related to males’ but not females’ PA, and positive peer interactions related to only females’ PA [F(1,329) = 9.58, p < .01]. Small, yet significant physical-environmental effects of temperature [F(1,328) = 1.54, p < .05] and equipment [F(1,328) = 4.34, p = .05] for girls also suggests that activities offered indoors (which was most common during high temperatures), and provision of equipment may also be important considerations for promoting girls’ PA. Staff behaviors were minimally predictive of youth PA.ConclusionsThis is the first study to conduct systematic observations of the physical and social resources of summer day camps and contributes to our understanding of the strengths and needs of camps to effectively promote PA in both boys and girls during the summer months when risks for obesity are high.