Childhood obesity trends have increased dramatically over the past three decade's. The purpose of this quantitative systematic review is to provide an update of the evidence, illustrating the efficacy of childhood obesity treatment, considering whether treatment fidelity has been measured and/or reported and whether this related to the treatment effect size. Searches revealed 61 relevant articles published from January 2000 to 2009, including both randomized and non-randomized controlled trials of childhood and adolescent obesity treatment interventions. The review identified scant measurement and reporting of issues around treatment fidelity, an increase in the number of interventions outside of the US, (particularly across Europe) and an emergence of studies involving older children and adolescents in a range of settings. Interventions comprising a dietary, exercise, and behavioral component, supported by family involvement and delivered by trained interventionists in specialized or supervised settings, appeared to offer a potentially effective treatment for obesity. However, concern remains over study quality (particularly sample size), dropout rates and study design. Variations in outcome measures and intervention designs means generalizable conclusions cannot be easily be made. Of greater concern is the lack of consideration for treatment fidelity, which has implications for the transfer of knowledge and the reliability of interventions. Conclusions from the review include; the need for increased accuracy of reporting and objective measurement of treatment fidelity; the need for further investigation of potential cost-effective treatment options (including maintenance strategies to enhance sustainability of current interventions); and an increase in the number of longitudinal trials beyond 1 year in duration.