Undergraduate research is viewed as an important catalyst for educational engagement and persistence, with an emphasis on the faculty mentoring relationship. Despite the common practice of having multi-tiered lab teams composed of newer undergraduates and more seasoned undergraduates serving as peer mentors, less is understood about the experience of peer mentors. Using the framework of legitimate peripheral participation, this study examined how peer mentors negotiated their roles in the lab. Nested case studies based on interviews with peer mentors, faculty members, and newer students illustrated how peer mentors establish credibility through prior lab experience and faculty-framed authority. Delegating supervision was an important component that helped newer students to accept the authority of the peer mentor. Implications for program development and future research involving peer mentoring are discussed.