Virtually every course of medical action is associated with some adverse risk to the patient. Discussing these risks with patients is a fundamental duty of physicians both to fulfill a role as trusted adviser and to promote the ethical principle of autonomy (particularly as embodied in the doctrine of informed consent). Discussing medical risk is a difficult task to accomplish appropriately. Challenges stem from gaps in the physician's knowledge about pertinent risks, uncertainty about how much and what kind of information to communicate, and difficulties in communicating risk information in a format that is clearly understood by most patients. For example, a discussion of the risk of undergoing a procedure should be accompanied by a discussion of the risk of not undergoing a procedure. This article describes basic characteristics of risk information, outlines major challenges in communicating risk information, and suggests several ways to communicate risk information to patients in an understandable format. Ultimately, a combination of formats (eg, qualitative, quantitative, and graphic) may best accommodate the widely varying needs, preferences, and abilities of patients. Such communication will help the physician accomplish the fundamental duty of teaching the patient the information necessary to make an informed and appropriate decision.