The Administration for Children and Families’ (ACF) Office of Planning, Research, and Evaluation (OPRE) contracted with Mathematica in the first two phases of YARH to provide evaluation technical assistance to grantees, support them in articulating and refining the design of their service models, assess the evaluability of each service model, and disseminate the knowledge developed. ACF is now in the third phase of YARH (or YARH-3) and is conducting a rigorous summative evaluation of a policy-relevant comprehensive service model developed and refined during the first two phases of YARH. The summative evaluation conducted under YARH-3 will examine the effect of Colorado’s Pathways to Success comprehensive service model (Pathways). Pathways is an intensive, coach-like case management model for youth and young adults in foster care. A large, cluster quasi-experimental impact study design will be used to test the effectiveness of Pathways in 37 counties in Colorado. The Pathways implementation study will support interpretation of the model’s impacts on outcomes and identify factors that contributed to or inhibited implementation of Pathways services in different counties; these findings will aid in the replication or improvement of future Pathways service delivery. The implementation study will systematically assess different contexts in which Pathways is being implemented and the fidelity to which Pathways is being implemented. This report describes the design of the Pathways implementation study.