ContextThe problems of doping in sport and the increasing use of nutritional supplements by athletes are issues that intersect to the degree that a large number of supplements may contain substances that are banned in sport. Many supplements contain substances that are associated with significant health hazards. Athletes consuming such supplement products may jeopardize their sporting status, and their health.ObjectivesTo clarify and summarize the current status of dietary supplements in general, and to describe specific problems that can be associated with supplement use so that sport physicians might be better prepared to address these issues with their athlete-patients.Data SourceAn analysis of recent and relevant literature accessed through MEDLINE, and interactions with clinicians, laboratory scientists, colleagues, and athletes.ConclusionsThe dietary supplement industry is completely unregulated in the United States; as a consequence, an abundance of supplement products of dubious value, content, and quality are now available around the world. It is known that many supplement products contain substances that are prohibited in sport—typically stimulants or anabolic steroid precursors. Many supplements contain substances (e.g., ephedrine) that have been associated with significant morbidity and mortality. Sport practitioners have particular responsibilities in addressing this issue. Athletes need to be aware of the problems that can follow supplement use, and sport authorities need to ensure that nutritional education and guidance for athletes is of the highest standard. The need for the appropriate regulation of dietary supplements is emphasized.