摘要:Background: A growing part of the efforts to promote student engagement and success in undergraduate STEM are the family of Student Support and Outreach Programs (SSOPs), which task undergraduate students with providing support and mentoring to their peers and near-peers. Research has shown that these programs can provide a variety of benefits for the programs’ recipients, including increased academic achievement, satisfaction, retention, and entry into STEM careers. This paper extends this line of inquiry to investigate how participation in these programs impacts the undergraduate STEM students that provide the mentoring (defined here as undergraduate mentor-teachers or UMTs). We use activity theory to explore the nature of metacognition and identity development in UMTs engaged in two programs at a public urban-serving university in the western USA: a STEM Learning Assistant program and a program to organize middle and high school STEM clubs. Constructs of metacognition and identity development are seen as critical outcomes of experiential STEM inreach and outreach programs. Results: Written reflections were collected throughout implementation of two experiential STEM inreach and outreach programs. A thematic analysis of the reflections revealed UMTs using metacognitive strategies including content reflection and reinforcement and goal setting for themselves and the students they were supporting. Participants also showed metacognitive awareness of the barriers and challenges related to their role in the program. In addition to these metacognitive processes, the UMTs developed their science identities by attaching different meanings to their role as a mentor in their respective programs and setting performance expectations for their roles. Performance expectations were contingent on pedagogical skills and the amount and type of content knowledge needed to effectively address student needs. The ability to meet students’ needs served to validate and verify UMTs’ role in the program, and ultimately their own science identities. Conclusion: Findings from this study suggest that metacognitive and identity developments are outcomes shaped not only by undergraduate students’ experiences, but also by their perceptions of what it means to learn and teach STEM. Experiential STEM inreach and outreach programs with structured opportunities for guided and open reflections can contribute to building participants’ metacognition and enhancing their science identities.