This study presents evaluates the effectiveness of the Healthy Opportunities for Physical Activity(HOP'N) After-School Program on preventing obesity in children. The investigators hypothesized that normal and overweight/obese children attending after-school sites randomized to the control condition will increase in weight status to a greater extent compared to children at sites randomized to receive the HOP'N program. The investigators also hypothesized that after-school intervention HOP'N sites will increase in physical activity and healthful eating opportunities compared to control sites.

The dramatic increase in the prevalence of overweight among children and adolescents has led to obesity prevention becoming a major national health priority. Although schools offer a logical choice for a setting to reach a great number of youth, these disappointing findings may be due in part to the difficulties of implementing interventions in school settings where competing demands for time have made it difficult to add anything other than academics to the school day. After-school programs may provide promise for preventing child obesity because there are fewer bureaucratic obstacles and curricular inflexibilities. A three-year group-randomized controlled trial will be conducted with random assignment at the school level . The study will use a nested cross section design with a baseline year, and two subsequent intervention years.

Inclusion Criteria:

• after-school program participant in fourth grade group
• informed parental consent
• assent to participate in Body Mass Index assessment

Exclusion Criteria:

• third or fourth grade student
• participant in study in previous year