Overconfidence bias refers to the systematic tendency to overestimate one’s own abilities, knowledge, and judgments, resulting in disproportionate confidence in the accuracy of personal decisions. This chapter illustrates the bias through the case of Benjamin Rush, one of the most influential physicians in the young United States, whose belief in his unifying theory of fever led him to employ widespread bloodletting and purging during the 1793 yellow fever epidemic in Philadelphia. Despite poor clinical outcomes and warnings from his colleagues, Rush maintained unwavering support for his method, even applying it to himself when he fell ill. The chapter reviews the definition, psychological, and historical foundations of the bias, from Oskamp’s pioneering studies to the research of Fischhoff, Slovic, Lichtenstein, and the later Kruger and Dunning model. It also analyzes the repercussions of the bias in personal and collective life, including hasty decision-making, risk underestimation, suppression of dissent, and the potential for catastrophic events. Its manifestations in medical and surgical practice are also examined, such as diagnostic errors, denial of complications, rejection of second opinions, and resistance to therapeutic innovations. Finally, strategies to mitigate this bias are proposed, including intellectual humility, critical feedback, a growth mindset, the use of pre-mortem analysis, and the systematic recording of predictions and errors.