Summer is a period of accelerated BMI gain for children (5-12yrs). Studies show that virtually all increases in BMI occur during the summer, no matter children's' weight status (i.e., normal weight, overweight, or obese) at summer entry. Our research team recently developed the Structured Days Hypothesis (SDH), which may explain accelerated summer BMI gain. The SDH posits that structure, defined as pre-planned, segmented, and adult-supervised compulsory environments, protect children against obesogenic behaviors and prevent excessive BMI gain. The SDH draws upon the 'filled-time perspective', which posits that time filled with favorable activities cannot be filled with unfavorable activities. In the context of the SDH, this means that children engage in more obesogenic behaviors that lead to increased weight gain during times that are less-structured (e.g., summer days) compared to times that are more structured (e.g., school days). Based on the SDH, pre-existing community-operated summer day camps (e.g., B\&G Club, YMCA), may exert a positive influence on summer BMI gain by limiting children's engagement in obesogenic behaviors. Recent preliminary studies show that children engage in healthier behaviors on days that they attend summer day camps, and that BMI gain does not accelerate for these children. A major weakness in the rigor of these preliminary studies is that they cannot identify the dose-response relationship between structured summer programming and summer BMI gain. Dose-response studies can definitively identify the smallest dose at which a useful effect is observed while simultaneously revealing the maximum dose beyond which there is no further beneficial effect. In the same way determining the effective dose of structure to mitigate negative health outcomes is necessary to inform feasible, scalable interventions and health policy. The proposed randomized dose-response study will identify the dose-response relationship between amount of summer programming and summer BMI gain. The impact of 4 weeks (n=90, 20 days), 6 weeks (n=90, 30 days), and 8 weeks (n=90, 40 days) of summer programming compared to no program (n=90) will be evaluated. Comprehensive implementation monitoring to evaluate implementation will also be conducted. This will identify factors associated with children's summer BMI gain and obesogenic behaviors. The aims of the study are to: Aim 1. Evaluate structured summer programming's impact on children's BMI gain and obesogenic behaviors. Aim 2. Evaluate implementation and contextual factors and their relationship with children's summer BMI gain and obesogenic behaviors. Aim 3. Determine the cost effectiveness of 4, 6, 8 weeks of summer programming for mitigating accelerated summer BMI gain. This work is significant as it addresses a critical public health goal - reducing obesity - through programing during a timeframe - summer vacation - when substantial, long-lasting negative effects occur. This application is innovative because of the focus on identifying the dose-response relationship between structured summer programming and summer BMI gain. This innovation addresses a weakness in the rigor of previous studies and is critical for identifying the ideal dose of summer programming for mitigating accelerated summer BMI gain.