Over the past few decades, mentoring, long an informal set of practices, has begun to crystalize into a profession. It is becoming not only a calling but also a career. This is due, in part, to the challenges presented by the increasingly complex environments of the youth who are being mentored today. Successfully assisting a youth in navigating through complexity requires both skills and nuance. In our experience, the level of professionalism required is unlikely to reside naturally in the typical volunteer mentor. The presence of a guiding hand—a mentor who is prepared and up to the task—and one who has the time to do so because their job is to be a mentor, may make all the difference in whether or not a youth can not only address the challenges in front of them but also seize the opportunities that arise along the way. This chapter introduces the Handbook of Professional Youth Mentoring, which brings together the writings of a wide variety of people who have spent their careers thinking about children, mentors, and the practice of mentoring. The goal of the handbook is to help the field better understand both the power of mentoring in general and when, how and with whom professional mentoring might be most useful.