Childhood obesity has a complex multi-factorial aetiology grounded in environmental and individual level factors that affect behaviour and outcomes. An ecological, systems-based approach to addressing childhood obesity is increasingly being advocated. The primary aim of this review is to summarize the evidence reported in systematic reviews on the effectiveness of population-level childhood obesity prevention interventions that have an environmental component. We conducted a systematic review of reviews published since 1995, employing a standardized search strategy in nine databases. Inclusion criteria required that reviews be systematic and evaluated at least one population-level, environmental intervention in any setting aimed at preventing or reducing obesity in children (5-18 years). Sixty-three reviews were included, ten of which were of high quality. Results show modest impact of a broad range of environmental strategies on anthropometric outcomes. Systematic reviews vary in methodological quality, and not all relevant primary studies may be included in each review. To ensure relevance of our findings to practice, we also report on relevant underlying primary studies, providing policy-relevant recommendations based on the evidence reviewed. Greater standardization of review methods and reporting structures will benefit policymakers and public health professionals seeking informed decision-making.