The University of Texas System established the Transformation in Medical Education (TIME) initiative to reconfigure and shorten medical education from college matriculation through medical school graduation. One of the key changes proposed as part of the TIME initiative was to begin emphasizing professional identity formation (PIF) at the premedical level. The TIME Steering Committee appointed an interdisciplinary task force to explore the fundamentals of PIF and to formulate strategies that would help students develop their professional identity as they transform into physicians. In this article, the authors describe the task force's process for defining PIF and developing a framework, which includes 10 key aspects, 6 domains, and 30 subdomains to characterize the complexity of physician identity. The task force mapped this framework onto three developmental phases of medical education typified by the undergraduate student, the clerkship-level medical student, and the graduating medical student. The task force provided strategies for the promotion and assessment of PIF for each subdomain at each of the three phases, in addition to references and resources. Assessments were suggested for student feedback, curriculum evaluation, and theoretical development. The authors emphasize the importance of longitudinal, formative assessment using a combination of existing assessment methods. Though not unique to the medical profession, PIF is critical to the practice of exemplary medicine and the well-being of patients and physicians.