Androgens have a dual effect on mammary cells. Indeed, they have an influence on mammary cells proliferation thanks to several possible mechanisms, including their transformation into dihydrotestosterone (5alpha-reductase pathway) or into estradiol (aromatase pathway) or their binding to the androgen receptor (AR) and/or to the estrogen receptor (ER). Androgen signaling, using 5alpha-reductase pathway, enables the control of cell proliferation, mediated by AR. So androgen signaling plays a crucial role in breast homeostasis, negating the proliferative effects of estrogen signaling in the breast. When androgens transform into estrogens (aromatase pathway), they increase cell proliferation and mammary carcinogenesis risk. High levels of androgens and estrogens in the serum are associated with increased incidence of postmenopausal breast cancers. Genetic variations in metabolic genes (CYP11, CYP19) and in the AR gene are both involved in dual effects of androgens. Since mammary cells metabolic enzymes vary with time, aging increases the risk of breast cancer induced by estrogens and androgens. In addition, AR function can be perturbed by low doses of synthetic progestin, acting as endocrine disruptors to negate the protective effects of androgen signaling in the breast. In the future, the determination of AR expression in infiltrative breast cancer specimens and circulating androgens levels could provide additional information about hormonal dependency and prognosis of breast carcinomas.