The delivery of undergraduate instruction at research universities is highly dependent on graduate teaching assistants (GTAs). The GTA position provides critical instructional support, as well as the pedagogical training and first-hand teaching experiences for future faculty. To date, most research on graduate student instruction pertains to evaluations of GTA teacher training programs. A small body of research documents self-reported changes in elements of GTAs' teaching practices (e.g., pedagogical approaches and beliefs) and teacher identities (e.g., instructional skills and style). These studies offer some insight on experiences that shape, as well as shifts that may occur within, GTAs' practices and conceptions of themselves as teachers. However, this research does little to clarify "how" GTAs learn to do the actual work of teaching or "why" evolutions to their instructional practices and identities occur. In this study, I depart from this earlier research to examine "how" and "why" GTAs' instructional experience shape their practices and identities as postsecondary instructors. This study was guided broadly by sociocultural learning theory and later included sociopolitical concepts. A primary focus of this research is how the contexts in which GTAs teach shape their evolving insights, practices, and identities as teachers. A secondary focus is the salience of GTAs' social identities and student experiences in their learning as teachers. Drawing on 27 semi-structured interviews, 18 audio journals, and 18 classroom observations, this exploratory, qualitative study takes an emic approach to understanding GTAs' learning and identity construction as first-time university instructors. The study sample consists of nine doctoral students, each new to the practice of university instruction and teaching in the social sciences at a research university. I chose to study GTAs without prior teaching experience to better understand how doctoral students new to the practice of postsecondary instruction learn about and begin to see themselves in relation to this work. Data collection for this study took place during and immediately after the semester in which GTAs were teaching. This study offers new insight on what GTAs learn, as well as how this learning occurs, through their instructional experiences. GTAs' insights about teaching pertained to their instructional environments and preparations; student learning, engagement, and conduct; and evaluation and assessment. GTAs' insights about themselves as teachers concerned their instructional qualifications, teaching style, and affinities towards teaching. I also discovered that GTAs' social identities and student experiences informed what they learned, the agency that they enacted, and how they perceived themselves as teachers. In addition, my analysis of the study data yielded a conceptual model, which offers a detailed visual of the potential mechanisms by which GTAs may learn, exercise agency, and form their own practices and identities as teachers. I present a set of theoretical propositions, based on this model and grounded in my data and the relevant literatures, to guide future research on GTAs' teaching-related learning and identity construction. I also outline several recommendations to support doctoral students' professional development as university 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.]