As synthetic derivatives of the male sex hormone testosterone, anabolic steroids are traditionally taken in cycles of 6 to 12 weeks duration. Some athletes, however, use them on relatively continuous basis and increase the dose at certain times to prepare for a competition. Athletes often take more than one steroid at a time, which is referred to as "stacking." The purported rationale for stacking is that the user will activate more receptor sites than if only one steroid is used. Athletes primarily use oral and intramuscular forms of AAS, and most who use AAS to enhance athletic performance and physical appearance obtain the drugs illicitly. The epidemiology of AAS use and drug testing requirements of the International Olympic Committee are reviewed, and surveys measuring the prevalence of AAS use are reported. 44 references and 5 tables