This study examines naturally occurring mentors by the quality and presence of a mentor (no mentor, low quality, high quality), type of mentors (adult mentors vs. peer mentors), and mentor quality within mentor type. A sub-sample of adolescents from the National Longitudinal Study of Adolescent-Health is used. Results indicate the effect of mentoring is dependent upon the quality and type of mentor and is different for different measured outcomes, as various characteristics of the mentoring process are associated with later problem behaviors and emotional affect in different ways. Specifically, high-quality mentoring is significantly related to increasing self-esteem, fewer alcohol problems and less depressed affect when compared to low-quality mentors. Mentees with adult mentors are significantly lower in depressed affect and have fewer alcohol problems when compared to mentees with peer mentors. Implications for the use of mentoring with specifically at-risk students are discussed to help improve positive youth development. (Contains 8 tables.)