Education continues to diversify with the growth of schools and the influx of beginning teachers. The increased presence of beginning teachers emphasizes the need for teacher-mentoring. This systematic review examined what research shows about training and preparation of teacher-mentors in high school education, revealing three themes: mentor services, mentor training, and where to begin mentor training. Key findings state that while significant amounts of research has been conducted on the support mentor teachers provide to mentees, there is limited research on how mentors are trained. Finally, recommendations and implications for further research on teacher-mentor training are provided.