The halo effect is a cognitive bias in which abilities, skills, or attributes of a person or situation are inferred based on a single striking characteristic. This chapter illustrates the phenomenon through the case of the Shah of Iran and the selection of renowned cardiovascular surgeon Michael DeBakey to perform a splenectomy, despite his lack of recent experience in abdominal surgery. DeBakey’s fame, prestige, and technical brilliance in his field served as a powerful perceptual anchor, leading the Shah’s medical team to assume that his excellence was transferable to any surgical procedure. The psychological foundations of the halo effect are reviewed, from Thorndike’s initial observations to later work by Asch and by Nisbett and Wilson on the influence of physical attractiveness and the unconscious nature of bias. The chapter also discusses its impact on the evaluation of people, products, and collective decisions, as well as its frequent manifestations in clinical reasoning, diagnosis, treatment, and medical communication. Finally, strategies are proposed to reduce the influence of bias, including deliberate neutrality, skepticism, objective assessments, multiple observers, and conscious recognition of the limitations of first impressions.