Professional identity has emerged as a common theme in teacher development research, and the student-teaching practicum is often identified as foundational to identity development. In the context of the student-teaching practicum, interactions with cooperating teachers and pupils are believed to comprise the press for professional identity development, though theory-based explanations are often neglected in the literature, and findings are not always consistent. To address this issue, we used grounded theory to articulate a model explaining the relations among three constructs important to the process of identity development of student teachers (n = 14). Our findings are organized around a model that highlights the phenomenon of "negotiating who I am as a teacher," which helps us describe differences between student teachers who changed identity vs. those that did not, and psychological and contextual reasons for renegotiation of identity. Discussion focuses on comparisons with previous models and possible implications for teacher education.