Formal mentoring is the cornerstone of most teacher induction programs, but teachers receive support from informal mentors as well. The authors report on findings from a study of 57 first-year mathematics teachers in 11 school districts to describe who teachers' formal and informal mentors are, how teachers work with them, and how the functions of formal and informal mentors serve for beginning teachers. They offer suggestions for how schools and districts can harness the power of these two forms of mentoring by embracing both.