Effectively scaling up evidence-based educational practices requires identification of barriers to the wide-scale implementation and maintenance of such practices at a distance from the original research and development team. The two studies reported here investigated the implementation of ClassWide Peer Tutoring (CWPT) in nine schools across five states. In Study 1, implementation was measured in terms of the percentage of teachers who completed the tasks required to conduct CWPT and the research protocol. Factors were identified that may have contributed to implementation variability across schools. In Study 2 we used "rate of implementation" as the implementation measure in an effort to assess the degree to which the factors identified in Study 1 affected implementation across a larger group of schools. Barriers were classified as Strong, Weak, or Non-Barriers. Implementation barriers included limited communication between practitioners and research staff, lack of support for CWPT from school and/or district leaders, and unexpected changes in school administrative responsibilities or staffing. Strategies for overcoming these barriers and the need for additional implementation research are discussed. (Contains 4 tables and 5 figures.)