This chapter reviews the rationale for implementing coaching as a key form of teacher professional development. It describes the impact of collaborative coaching and learning (CCL) on teaching and learning in Boston Public Schools (BPS) and considers the factors that affected whether and how the model was implemented in those settings. As Boston's schools worked to implement CCL, the factors that influenced their success include principal leadership, coach knowledge and skill, teachers' attitudes, knowledge, and skill. The factors also include the social context/climate of the school, school organization and size, and district support. The chapter elaborates these factors and discusses the ways in which adaptations to the CCL model, made in response to these factors, influenced the fidelity and quality of CCL implementation. Coaching, organized as CCL, holds out the promise of achieving this goal by fundamentally reforming teaching as a culture as well as a skilled practice.