BACKGROUND: Multisource feedback (MSF) is emerging as a central assessment method for several medical education competencies. Planning and resource requirements for a successful implementation can be significant. Our goal is to examine barriers and challenges to a successful multisite MSF implementation, and identify the benefits of MSF as perceived by participants. METHODS: We analyzed the 2007-2008 field trial implementation of the Assessment of Professional Behaviors, an MSF program of the National Board of Medical Examiners, conducted with 8 residency and fellowship programs at 4 institutions. We use a multimethod analysis that draws on quantitative process indicators and qualitative participant experience data. Process indicators include program attrition, completion of implementation milestones, number of participants at each site, number of MSF surveys assigned and completed, and adherence to an experimental rater training protocol. Qualitative data include communications with each program and semistructured interviews conducted with key field trial staff to elicit their experiences with implementation. RESULTS: Several implementation challenges are identified, including communication gaps and difficulty scheduling implementation and training workshops. Participant interviews indicate several program changes that should enhance feasibility, including increasing communication and streamlining the training process. CONCLUSIONS: Multisource feedback is a complex educational intervention that has the potential to provide users with a better understanding of performance expectations in the graduate medical education environment. Standardization of the implementation processes and tools should reduce the burden on program administrators and participants. Further study is warranted to broaden our understanding of the resource requirements for a successful MSF implementation and to show how outcomes change as MSF gains broader acceptance.