Loss aversion is a cognitive bias describing the human tendency to perceive losses as more emotionally intense and decisive than equivalent gains, leading to decisions that favor security over potential benefit. This chapter illustrates the phenomenon through the prolonged controversy surrounding the recognition of vascular surgery as an independent specialty versus general surgery in the United States. Although accumulating evidence demonstrated better clinical outcomes in the hands of specialists, higher surgical volumes, and a growing need driven by an aging population, the perception of “loss” of training autonomy, case volume, or professional identity generated sustained resistance among some general surgeons. The chapter reviews the psychological and historical foundations of loss aversion, from Kahneman and Tversky’s prospect theory to contemporary research examining its relationship with emotions, risk, status, and social behavior. It also analyzes its impact on innovation, adoption of new practices, framing of alternatives, and institutional dynamics. Finally, clinical and surgical applications of bias are discussed, along with practical mitigation strategies such as extending the time horizon, analyzing complete decision portfolios, involving neutral third parties, and making decision-making responsibility explicit.