This post is one in a series highlighting MDRC's methodological work. Contributors discuss the refinement and practical use of research methods being employed across MDRC. MDRC has begun working with research partners through the Expanding Children's Early Learning (ExCEL) Network, a collaboration of researchers, preschool providers, and local stakeholders, to construct tools to assess implementation fidelity in a set of early childhood education programs developed by Boston Public Schools. This report discusses the three stages of the work conducted by the research partners, as well as the benefits and challenges of the partnership. [For the second report in this series, see "Linking Fidelity of Implementation to Outcomes in Real-World Settings. Reflections on Methodology" (ED601202).]