Mentor teachers (MTs) play a key role in a teacher candidate's (TC) learning to teach and teaching a TC to become a teacher is complex work. While there has been some scholarship that indicates MTs benefit from collective inquiry into and examination of their mentoring practice, there is minimum attention to engaging MTs in deliberate activity about their understanding and improving their practice as mentors. The purpose of this qualitative study was to answer the following research questions: 1. How do the dual responsibilities MTs have shape their mentoring practice? 2. In what ways does a collective reflection and inquiry group support MTs in addressing the problems of practice they encounter, including tensions they experience between serving as MTs and classroom teachers? 3. What are the limitations of an MT reflection and inquiry group? From October 2017 through March 2018 I met monthly with six high-school MTs from four content areas with one to 15 years of mentoring experience. Main qualitative data sources included audio-recordings of the monthly meetings, as well as periodic interviews with each MT and field notes of two full-day observations of each MT-TC pair in their classrooms. Drawing on sociocultural theories of identity, specifically practice-linked identities and multimembership in relation to reflection in and on action, I found that, consistent with other research, the MTs struggled to reconcile their commitments as MTs with those as classroom teachers. Namely, while MTs engaged in explicit mentoring outside of classroom instruction, I did not observe explicit mentoring during instructional time in the presence of students. This tension, among others, was discussed in the MT reflection and inquiry group. Analyses of group conversation and interviews with MTs showed that while MTs articulated benefits from collective reflection-on-action in the group, the group could not offer sufficient ways to solve the identity tensions MTs experienced. Namely, collective reflection-on-action resulted in more in-depth engagement with the work of mentoring, normalized problems of practice, and lessened the solitude inherent in mentoring. At the same time, reflection-on-action was not enough to solve the frustration MTs experienced during instructional time when their identities as classroom- and mentor teachers were conflicting with one another. These findings suggest that teacher education programs need to gain a more complete understanding of the ways challenges shape the MTs' mentoring practice and facilitate reconciliation of the tensions MTs experience, for example through supporting MTs in identifying opportunities for reflection-in-action during instructional time. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]