Breast cancer is the most common cancer among women in the United States. Knowledge of the descriptive epidemiology of breast cancer is useful both in suggesting etiologic hypotheses and, if preventive measures can be identified, in delineating high-risk groups to be targeted for preventive efforts. Demographic risk factors include increasing age (in Western countries), being white for breast cancer diagnosed at age 45 years or more, being black for breast cancer diagnosed at less than 40 years of age, high socioeconomic status, having never married, being of the Jewish faith, urban residence, and residence in the northern (as compared with the southern) United States. Incidence rates are generally highest in North American and Northern European countries, intermediate in Southern and Eastern European and South American countries, and lowest in Asia and Africa. The most notable characteristic of the descriptive epidemiology of breast cancer in recent years is perhaps the rapidly increasing incidence rates in developing countries. Identification of specific reasons for these increasing rates would contribute substantially to our understanding of the epidemiology of breast cancer.