The ultimate goal of breast cancer prevention strategies is to reduce the incidence of this disease in populations. Greater understanding of recently identified associations of lactation, alcohol, exercise, and diet with breast cancer is necessary to bring these to bear favorably on the behavior of populations. As a hormonally related process, breast cancer incidence is associated with two major physiologic mechanisms: (1) extent of lobular maturation, which is profoundly influenced by the occurrence of a full-term pregnancy, and (2) hormonal exposure of the breast epithelium, which is influenced by a spectrum of lifestyle factors. Manipulation of these processes by technologically simple and practical means is a major goal of research. Modulation of preclinical growth of breast cancers by chemopreventive means poses significant challenges, due to the absence of target-organ specificity and frequent toxicity. With the emergence of well-supported models of breast cancer development, behavioral and social strategies are likely to be key to achieving the ultimate goal.