Although calls for a more diverse workforce in biomedical fields have been widespread, racial and ethnic gaps in biomedical degree attainment remain. Contextualist perspectives seek to understand persistent STEM inequities by examining person-in-context experiences and how systemic factors filter into students' proximal contexts shaping their participation and science identity trajectories. Research training communities of practice aim to offer underrepresented minority and first-generation students support, guidance, and opportunities to learn the practices of science and construct their science identity. However, many students still choose to leave these programs. There is limited research on these students' science identity construction process and their identity trajectories. This study fills this gap by examining contextual factors shaping participation, identifies essential experiences shaping student science identity construction, and explores the link between science identity and decisions to leave research training communities of practice. Semi-structured interviews were conducted with 23 underrepresented racial and ethnic minority and/or first-generation students participating in a research training community of practice. Twelve participants completed the program, and eleven left the program. Through thematic analysis, several themes were identified. Study results revealed the central role of contextual factors including college affordability, racialized dynamics in STEM, scientific norms that impact student well-being, narrow pedagogical approaches, and the program's motivational climate in shaping students' participation and departure decisions. Study results also highlighted the importance of explicit inclusion in meaningful science practice, legitimate peripheral participation, and scaffolded mentoring as they learned scientific practices. Additionally, the study highlighted the importance of performance, competence, recognition, within the research training community of practice and the centrality of marginalized identities in the science identity construction process. This study provides critical insight into the underrepresented racial and ethnic minority and first-generation STEM student experience, the science identity construction process, and contextual factors contributing to choices to depart from research training programs and/or STEM career pathways. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]