Over the course of training, physicians develop significant knowledge and expertise. We review dual-process theory, the dominant theory in explaining medical decision making: physicians use both heuristics from accumulated experience (System 1) and logical deduction (System 2). We then discuss how the accumulation of System 1 clinical experience can have both positive effects (e.g., quick and accurate pattern recognition) and negative ones (e.g., gaps and biases in knowledge from physicians' idiosyncratic clinical experience). These idiosyncrasies, biases, and knowledge gaps indicate a need for individuals to engage in appropriate training and study to keep these cognitive skills current lest they decline over time. Indeed, we review converging evidence that physicians further out from training tend to perform worse on tests of medical knowledge and provide poorer patient care. This may reflect a variety of factors, such as specialization of a physician's practice, but is likely to stem at least in part from cognitive factors. Acquired knowledge or skills gained may not always be readily accessible to physicians for a number of reasons, including an absence of study, cognitive changes with age, and the presence of other similar knowledge or skills that compete in what is brought to mind. Lastly, we discuss the cognitive challenges of keeping up with standards of care that continuously evolve over time.