All editorial matter in CMAJ represents the opinions of the authors and not necessarily those of the Canadian Medical Association. Commentary

The rising prevalence of childhood obesity in most de-veloped countries is well recognized,1 as are themany short- and long-term complications that can occur as a result.2 The past decade has seen a rapid growth in research into various aspects of this epidemic. However, in most countries, there has been a far slower response in terms of the number of funded policies and services aimed at preventing and treating childhood obesity. What strate-gies should be implemented? In this issue of CMAJ, Harris and colleagues3 report the re-sults of a systematic review of randomized controlled trials and controlled clinical trials designed to determine the effect of school-based physical activity interventions on body mass index among children.3 The 18 included studies mainly tar-geted elementary school children and covered a range of in-terventions, including moderate to vigorous physical activity, increased school time for exercise, and weight bearing exer