This study explored whether or not, and how, an on-site and research-teacher community of practice model for professional development addressed the challenges to classroom practices in a Head Start program. Data sources included interviews with teachers, videos of planning and teaching sessions, and the researchers' fieldwork log and reflective notes. Qualitative analysis revealed (a) five major challenges to classroom practice that aligned with previous research--existing practices did not always cohere with research-based practice, lack of teacher buy-in, lack of planning between the lead and assistant teachers, lack of assistant teacher participation in instruction, and high teacher turnover; (b) five aspects of professional development that aligned with the community of practice model-sharing stories with teachers, being on-site and interacting with teachers and children, inviting all teachers to participate, beginning by building on existing practices, and negotiating practices by reflecting with teachers; and (c) how these aspects of our professional development model addressed the challenges to classroom practice in the focal classroom. Implications for in-service professional development and teacher education are discussed.