The framing effect is a cognitive bias in which the way information is presented influences decision-making more than objective information itself. This chapter illustrates the phenomenon through the historical confrontation between William Halsted’s radical mastectomy and the breast-conserving surgery proposed by Bernard Fisher in the mid-twentieth century. Although evidence from NSABP clinical trials demonstrated that less extensive surgery offered equivalent oncological outcomes with lower morbidity, the paradigm shift was not immediate. The initial resistance was largely due to how the data were framed: Halsted’s proponents emphasized the risks of leaving residual tumor and invoked the authority of the “father of American surgery,” while Fisher presented the same data by highlighting overall survival, aesthetic preservation of the breast, reduced functional sequelae, and the strength of the scientific method over individual prestige. The chapter reviews the definitions, classifications, and psychological foundations of framing, from Kahneman and Tversky’s prospect theory to Reyna and Brainerd’s fuzzy-trace theory, and analyzes how this bias operates in clinical practice, marketing, medical communication, and surgical decision-making. Finally, strategies to mitigate framing are discussed, including in-depth subject knowledge, use of structured decision-making tools, peer review, analysis of complementary information, and formulation of multiple alternatives to avoid false dilemmas.