In the era of evidence-based medicine (EBM), discussions of medical knowledge have largely focused on the role of clinical research, though even the primary architects of EBM have acknowledged that the results of such research should not alone dictate decisions about care for individual patients. The most widely cited definition of EBM states that knowledge from clinical trials must be integrated with a physician's clinical expertise, knowledge of physiology, and understanding of their patient's values. Yet proponents of EBM have provided sparse guidance on how this integration should occur. On What Evidence? aims to solve this "integration problem" by considering how clinicians gain and demonstrate expertise, what kinds of medical knowledge can legitimately be brought to bear, and how knowledge of individual patients should be obtained and evaluated in clinical decision-making. The authors analyze the types of knowledge necessary to provide effective care and examine the medical and philosophical literature on each. Fusing real-life practice and theoretical rigor, On What Evidence? describes and defends a case-based approach to clinical decision-making, one based on a broad and non-hierarchical view of medical epistemology.