The anabolic-androgenic steroids (AAS) are a family of hormones that includes the natural male hormone, testosterone, together with a group of synthetic derivatives of testosterone. These drugs are widely abused by men (and rarely, women) to gain muscle mass and lose body fat. Prior to about 1980, abuse of AAS was con fi ned largely to elite competitive athletes, but in recent decades, AAS abuse has broken out of the athletic community and into the general population. Many modern AAS users have no speci fi c athletic aspirations at all, but simply want to become bigger and more muscular. About 2-6% of men in many Western industrialized countries have used AAS, but AAS use is rare in Asian societies. Individuals with body image concerns, such as "muscle dysmorphia," appear more prone to abuse AAS. Male muscularity is more strongly emphasized and rewarded in industrialized Western cultures than in Asia, and this difference likely explains the geographic distribution of AAS abuse. AAS cause few serious short-term medical effects, but over the long term may cause suppression of hypothalamic-pituitary-gonadal function, adverse effects on serum lipids, and cardiomyopathy. The most common psychiatric effects of AAS are mood disorders (typically hypomanic or manic syndromes during AAS exposure and depressive symptoms during AAS withdrawal); these are idiosyncratic, affecting a minority of AAS users, but are occasionally severe. A growing literature describes syndromes of AAS dependence, where individuals use AAS almost continuously despite adverse medical or psychiatric effects. Individuals displaying AAS abuse or dependence may also exhibit other forms of substance dependence. Unfortunately, AAS users rarely seek treatment, but this situation may change as the fi rst large wave of illicit AAS users-those who fi rst began AAS as youths in the 1980s-now reaches middle age and enters the age of risk for long-term cardiac, neuroendocrine, and psychiatric complications from these drugs.