The authors draw attention to the need to introduce the concept of epistemological beliefs into medical education. Epistemological beliefs are the cognitions (i.e., understandings) individuals have on knowledge and knowing and determine how (new) knowledge is perceived and processed. Within educational psychology, three different frameworks have been used to study epistemological beliefs. The authors describe these three frameworks and discuss their applicability and their significance for phrasing and studying issues that intuitively seem essential to medical educators. The metaphor of a piloted hot-air balloon illustrates the different factors contributing to medical expertise: the hot-air balloon's basket symbolizes the well-organized knowledge base, the envelope (i.e., air bag) stands for the skills repertoire of the pilot, and the burners represent motivation, intelligence, and other noncognitive factors. The pilot needs to achieve sophisticated levels of epistemological beliefs and metacognitive skills to be able to reach the upper levels of expertise with his well-equipped balloon. The metaphor emphasizes the dynamic disposition of expertise and offers a visual framework for designing curricula, assessment procedures, and educational research projects. Future research into medical students' epistemological beliefs should focus on outcome measures that are relevant within the medical education setting, and must take into account a number of pitfalls and difficulties inherent both in the concept of epistemological beliefs and in research in medical education.