More students with more severe mental health conditions are entering college, putting ever greater pressure on university resources to meet the needs, both psychological and educational, of these students. Graduate student instructors and TAs (TAs) are uniquely positioned to connect these students to supportive resources, but may be underprepared and have attitudinal barriers to addressing student mental health concerns. The goal of this study was to examine TA attitudes, knowledge, and self-efficacy for addressing student mental health concerns and offer suggestions for potential TA training targets. Methods: Vignettes of three levels of student conflicts with an exam were piloted on a sample of former TAs. Results from this pilot were used to select a final set of three vignettes to use in the main study. In the main study, current TAs freely responded to "emails" depicting three levels of student exam conflicts (control, transient, and chronic) as well as survey items measuring mental health knowledge, attitudes, gatekeeping self-efficacy, job and TA characteristics. Qualitative methods were used to generate themes regarding referral and accommodation of students as well as to identify organizational and emergent themes regarding how TAs address student mental health concerns. Quantitative methods were used to examine how TA individual characteristics relate to TA cognitions and gatekeeping behavior as well as potential moderating effects of situation and department context. Results: Three vignettes were finalized using pilot data. This study provided an in-depth description of TAs, their training, and their experiences responding to student MH problems. It further provided partial support for the conceptual model and identified potential training gaps. Conclusion: This study used a mixed methods approach to provide a revised conceptual model of TA behavior. A combination of TA personal characteristics, mental health knowledge, and context are associated with TAs' referral and accommodation behaviors, which vary in degree of appropriateness. Recommendations based on this model include filling gaps in TA training focusing on professional development, student mental health, and ethics training tailored to the unique challenges of graduate student instructors. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]