The clinician-scientist role is critical to the future of health care, and in 2010, the Carnegie Report on Educating Physicians focused attention on the professional identity of practicing clinicians. Although limited in number, published studies on the topic suggest that professional identity is likely a critical factor that determines career sustainability. In contrast to clinicians with a singular focus on clinical practice, clinician-scientists combine two major disciplines, clinical medicine and scientific research, to bridge discovery and clinical care. Despite its importance to advancing medical practice, the clinician-scientist career faced a variety of threats, which have been identified recently by the 2014 National Institutes of Health Physician Scientist Workforce. Yet, professional identity development in this career pathway is poorly understood. This Perspective focuses on the challenges to the clinician-scientist's professional identity and its development. First, the authors identify the particular challenges that arise from the different cultures of clinical care and science and the implications for clinician-scientist professional identity formation. Next, the authors synthesize insights about professional identity development within a dual-discipline career and apply their analysis to a discussion about the implications for clinician-scientist identity formation. Although not purposely developed to address identity formation, the authors highlight those elements within clinician-scientist training and career development programs that may implicitly support identity development. Finally, the authors highlight a need to identify empirically the elements that compose and determine clinician-scientist professional identity and the processes that shape its formation and sustainability.