PROJECT SUMMARY/ABSTRACT Nearly one in five children in the United States are affected by obesity. Beyond the adverse impact on physical, social, and emotional wellbeing in childhood, children with obesity are at greater risk of obesity in adulthood. A recent simulation study projects that 57% of today’s children will have obesity at age 35, placing them at higher risk of cardiometabolic disease, obesity-related cancers, and early mortality. Effective interventions that prevent and treat obesity in childhood can disrupt these projected trends, improve long-term health and quality of life, and reduce later health care utilization and costs. Bright Bodies, a high-intensity, family-based intervention for childhood obesity, was found to have the greatest magnitude of mean reduction in body mass index among U.S.-based childhood obesity interventions evaluated in the 2017 U.S. Preventive Task Force Report. There is tremendous potential for population-level impact on obesity outcomes if Bright Bodies is successfully scaled-up nationally, yet little is known about adoption, implementation, and effectiveness at the 39 sites to which the Bright Bodies curriculum has been disseminated over the last decade. Through this award, we aim to capitalize on advances in the field of implementation science to conduct an observational evaluation of the naturalistic dissemination and implementation of the Bright Bodies intervention. The results of this mixed methods study of historical dissemination sites will be used to optimize the Bright Bodies implementation package though user-centered design methods. The newly-optimized package will be prospectively implemented at three heterogenous sites reflecting low-income populations with racial/ethnic, rural-urban, and geographic diversity. The scientific objectives of the proposed award are to conduct a hybrid type 2 effectiveness-implementation and cost-effectiveness study in which we will employ multiphase mixed methods to assess implementation processes and effectiveness outcomes simultaneously while also examining contextual factors that may mediate and modify implementation impact and effectiveness. To achieve these aims, we have assembled a transdisciplinary team with expertise in obesity treatment, implementation science, mixed methods, community engaged research, cost-effectiveness analysis, and user- centered design. The results from this study will provide an optimized, user-centered implementation package for Bright Bodies as well as valuable evidence on the implementation and effectiveness of the intervention in different settings to facilitate broad-scale dissemination and to enhance sustainability.