For the past 12 years, the author has worked with peer coaches in more than 40 countries, and has seen schools where peer coaches have collaborated with colleagues to improve teaching and learning in classrooms across the school. In other schools, coaches have played a critical role in creating a culture of collaboration that helps build the school's collective capacity to improve teaching and learning. In many schools, however, coaching is yet another small-scale, short-lived educational experiment. Explaining the differences in success is key to understanding how to implement coaching successfully. Successful coaches know their effectiveness in collaborating with peers to improve teaching and learning hinges on the support of principals who control the budget and other key resources. Many coaches also understand that the leadership they provide plays an important role in creating the support needed to sustain coaching. Their accomplishments, and their abilities to communicate them to their principal and colleagues, are essential to support and expand coaching in a school. In this article, the author discusses how successful coaching is a result of an interdependent relationship between the principal and coaches.