Graduate teaching assistants (GTAs) are an important teaching resource to universities. Most GTAs enter graduate school with limited if any teaching experience but are given the responsibility of teaching numerous sections of introductory courses. GTA professional development (PD) is essential for the formation and preparation of these novice teachers who may become future faculty. Most GTA studies have primarily focused on doctoral programs. The current study assessed a GTA PD course at a master's-level program that was developed to equip GTAs to teach the introductory biology labs at the study site. The purpose of this study was to identify which components of the GTA PD course serve as sources to improve the teaching self-efficacy of master's-level biology GTAs. This study was conducted as a mixed methods case study using an explanatory sequential mixed methods design. GTAs enrolled in the GTA PD course at the study site volunteered as participants. Participants completed a survey for the quantitative phase of the study. A subset of these participants was purposefully selected for the qualitative interviews to obtain two strata of GTAs that had higher or lower teaching self-efficacy. The instructor's perspective was captured as an additional source of data by conducting a document analysis on the change facilitator's guide which detailed the earlier account of the researcher's perspective of the GTA PD course years prior to the current study being proposed. The conceptual framework was based on social cognitive theory including the sources of self-efficacy along with situational leadership to help identify components of GTA PD that impact the teaching self-efficacy of GTAs. Findings of this study suggest integrating GTA PD into the weekly lab preparatory meetings to provide clear expectations and context specific pedagogy that GTAs were expected to use to teach the labs were major components to the PD course. GTAs gaining mastery experiences by teaching labs repetitively was one of the most important components of their training. The lab coordinator modeling how to teach the labs, facilitating collaborative discussions during weekly course meetings, and providing autonomy to GTAs gradually through their progression of the one-year GTA PD course were also essential components that improved the teaching confidence of GTAs. Findings from this study corroborate with previous research in that GTA PD needs to exceed one semester of training to substantially improve GTA teaching self-efficacy. Peer and mentor feedback were not as impactful on GTA PD as reported in the literature but may have been due to the nature of how it was unstructured in the present study. Results from the present study can inform stakeholders such as department heads, graduate faculty, and lab coordinators of the essential components needed to provide quality GTA PD to equip GTAs as effective teachers. This study suggests that support from department heads and graduate faculty is critical to the success of a GTA PD program. This study may also serve as a model to lab coordinators of how to structure and deliver their GTA PD using situational leadership. Specifically, findings from the present study can provide insight to lab coordinators of how they can utilize situational leadership to provide the gradual teaching autonomy GTAs need to develop into competent and confident teachers. Findings from this study corroborate with previous research which encourages GTAs that their time invested in teaching and research can be synergistic to their professional development. This is especially important for those GTAs who aspire to work in academia where they will likely be required to teach. Therefore, participating in quality GTA PD could provide critical training for GTAs to become valuable assets in academia who have a positive impact on student learning. [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.]