There is growing interest in shared medical decision making among patients, physicians, and policy makers. This requires patients to interpret increasing amounts of medical information, much of which is uncertain. Little is known about the optimal approaches to or outcomes of communicating uncertainty about the risks and benefits of treatments. The authors reviewed the literature on various issues related to uncertainty in decision making: conceptualizing uncertainty, identifying its potential sources, assessing uncertainty, potential methods of communicating uncertainty, potential outcomes of communicating uncertainty, and current practices and recommendations by expert groups on communicating uncertainty. There are multiple sources of uncertainty in most medical decisions. There are conceptual differences in how researchers define uncertainty and its sources, as well as in its measurement. The few studies that have assessed alternate means of communicating uncertainty dealt mostly with presenting uncertainty about probabilities. Both patients' and physicians' interpretation of and responses to uncertainty may depend on their personal characteristics and values and may be affected by the manner in which uncertainty is communicated. Research has not yet identified best practices for communicating uncertainty to patients about harms and benefits of treatment. More conceptual, qualitative, and quantitative studies are needed to explore fundamental questions about how people process, interpret, and respond to various types of uncertainty inherent in clinical decisions.