Little research has focused carefully on the means by which peer mentors foster development in undergraduate protégés. Two faculty members developed a holistic, peer-mentoring project in which 26 upperclassmen mentored 74 underclassmen at a midsize, 4-year institution. Mentor journal notes, open-ended protégé responses, and participant observations were analyzed using a grounded theory approach, resulting in the emergence of 7 themes of mentor service activity. Findings expand our understanding of the mechanics through which peer mentors successfully serve and foster the development of early undergraduates in making the transition to college. Results can be used to facilitate the transition of academically underprepared undergraduates, who have the lowest retention rates.