Obesity is a major public health problem affecting more than 12 million (~17%)U.S. children. The scientific community agrees that tackling this problem must begin in childhood to reduce risk of subsequent development of cardiovascular diseases and other chronic diseases. The Childhood Obesity Prevention and Treatment Research (COPTR) Consortium, initiated by the National Institutes of Health (NIH), is conducting intervention studies to prevent obesity in pre-schoolers and treat overweight or obese 7–13 year olds. Four randomized controlled trials plan to enroll a total of 1,700 children and adolescents (~ 50% female, 70% minorities), and are testing innovative multi-level and multi-component interventions in multiple settings involving primary care physicians, parks and recreational centers, family advocates, and schools. For all the studies, the primary outcome measure is body mass index; secondary outcomes, moderators and mediators of intervention include diet, physical activity, home and neighborhood influences, and psychosocial factors. COPTR is being conducted collaboratively among four participating field centers, a coordinating center, and NIH project offices.