This chapter demonstrates the potential utility of Transformational Leadership theory (TFL) for understanding and facilitating high-quality coachathlete relationships, and ultimately, for fostering Positive Youth Development (PYD) in sport. It begins with an overview of a framework that outlines the elements necessary for PYD in sport, and positions quality relationships within this framework. The chapter specifically focuses on coachathlete relationships and presents TFL as a valuable framework for PYD research. It illustrates how TFL theory may be useful for both understanding the associations between coaching behaviors and PYD and supporting the development of interpersonally focused coach education. To highlights the utility of TFL, it also outlines its practical application to sport coaching, and lay out an agenda for conducting interventions and research that may advance our understanding of the influence of coachathlete relationships on PYD in sport.