Purpose: The majority of children do not participate in sufficient amounts of daily, health-enhancing physical activity (PA). One strategy to increase activity levels is to promote PA within the after-school setting. The effectiveness of this strategy, however, remains unclear. A systematic review was conducted summarizing the research conducted to date regarding the effectiveness of after-school programs to increase PA. Methods: Three primary databases (EBSCO Host, PubMed, Science Direct), and citations from published reviews and included papers were searched for articles published between 1980 and February 2008. Articles were included that met the following criteria: findings specific to an after-school intervention in the school setting; youth (≤ 18yrs); the primary component or one of the components of the intervention was to promote PA; and reported outcome measures of PA, related constructs (self-efficacy), and/or physical fitness. Study outcomes were distilled into 6 domains related to PA: PA; physical fitness; body composition; blood lipids; psychosocial constructs; and sedentary behaviors. Analysis/Results: Effect sizes (ES) were pooled within studies and across studies for each domain, separately. Of the 797 articles found, 13 unique articles describing findings from 11 after-school PA interventions were reviewed. While all studies indicated PA as a main component of the tested interventions, only 8 studies measured PA. Nine of the studies were randomized controlled trials. The median sample size was 116 (range 21 to 1044) with an average duration of 26.9wks (range 9 to 96). From the 6 domains, ES favoring the interventions were demonstrated for PA (ES 0.33, 95CI 0.21-0.44), physical fitness (ES 0.16, 95CI 0.02-0.29), body composition (ES 0.08 (95CI 0.04-0.12), and blood lipids (ES 0.20, 95CI 0.06-0.34). Conclusions: The limited evidence suggests after-school programs can improve PA levels and other health-related aspects of youth. Unfortunately, the considerable heterogeneity of programming and targeted population, coupled with lack of sufficient detail on the types of activities students engaged during the after-school program, make it difficult to determine the active ingredients of a successful after-school program. Nevertheless, it appears that attendance rates and intensity level of the physical activities provided play a role in positive outcomes. Issues related to transportation and age-appropriate activities of moderate-to-vigorous intensity attendees enjoy are areas of required attention to maximize program effectiveness. Future research should afford greater consideration to these issues, along with theoretical rationale, detailed description of implementation and program activities, and measures of PA within and outside the intervention.Keyword(s): health education K-12, health promotion, research