Although the research literature on mentoring continues to grow, and much has been learned about the influence of formal and informal mentor relationships on youth and early adult development, less is known about athletic coaches as mentors and how coach–athlete relationships influence short- and long-term human development. This is a bit surprising, given that participation in individual and team sports is one of the most common extracurricular activities for youth. Youth sports can be a context for promoting key social and emotional skill development, such as empathy and problem solving and for developing life skills such as mastery, persistence, and self-discipline. Further, youth involvement in sports creates the opportunity for developing strong, caring, and supportive relationships with adults. Gaining a fuller understanding of aspects of mentoring in sport—such as the qualities and characteristics of effective coach–athlete relationships, the ways coaches may act as formal and informal mentors, and how coach–athlete relationships influence the positive outcomes of youth sport participation—has the potential to enhance the positive effects of sports on youth development. In this chapter, an argument is made that it is long past time to professionalize the mentoring work of youth athletic coaches through high-quality, evidence-based training, supervision, and support, and to be able to expect that each and every coach who works with a child will play a constructive and positive role in their development.