Background Obesity prevention is an international public health priority. The prevalence of obesity and overweight is increasing in child populations throughout the world, impacting on short and long-term health. Obesity prevention strategies for children can change behaviour but efficacy in terms of preventing obesity remains poorly understood. Objectives To assess the effectiveness of interventions designed to prevent obesity in childhood through diet, physical activity and/or lifestyle and social support. Search strategy MEDLINE, PsycINFO, EMBASE, CINAHL and CENTRAL were searched from 1990 to February 2005. Non-English language papers were included and experts contacted. Selection criteria Randomised controlled trials and controlled clinical trials with minimum duration twelve weeks. Data collection and analysis Two reviewers independently extracted data and assessed study quality. Main results Twenty-two studies were included; ten long-term (at least 12 months) and twelve short-term (12 weeks to 12 months). Nineteen were school/preschool-based interventions, one was a community-based intervention targeting low-income families, and two were familybased interventions targeting non-obese children of obese or overweight parents. Six of the ten long-term studies combined dietary education and physical activity interventions; five resulted in no difference in overweight status between groups and one resulted in improvements for girls receiving the intervention, but not boys. Two studies Interventions for preventing obesity in children (Review)