Erectile dysfunction (ED) affects millions of men throughout the world. The literature is ample, but an accurate estimate of its prevalence is still difficult since figures mainly refer to the USA, and are based mostly on small selected samples of people. Caution must anyway be used in comparing data from studies conducted in the past because of possible differences in the definition and classification of ED. Many factors are believed to contribute to the development and maintenance of ED. The influence of age and of several medical conditions (diabetes, vascular disease, and chronic diseases such as hepatic failure, renal failure and dialysis) is well defined. Also well documented is the role of some drug groups, whereas the role of other pharmacological agents is still controversial because of the frequent coexistence of other pathological conditions or concomitant exposure to other drugs. Less well defined and sometimes controversial is the role of risk factors mainly related to life-style such as cigarette smoking, alcoholism, total cholesterol levels and certain types of trauma. This paper reviews the main data on the epidemiology of ED and some related risk factors.