Numerous studies have examined the risk of breast cancer in patients with postmenopausal hormone substitution. Most of these studies are retrospective, and a few recent studies are prospective. The observed results present with weak variations from baseline and major heterogeneity. Some studies highlight a slightly increased relative risk of breast cancer. A reanalysis of 51 studies demonstrates a relative risk of 1.35 for developing breast cancer during hormone substitution, with a 2.3% increased risk per year of use. Recently, the results of the WHI study have shown a slight increase of some risks of disease, including breast cancer (relative risk, 1.26). These results have induced the interruption of one of the 3 arms of the study (that of the patients treated with an estrogen-progestin combination), and have provoked a new discussion about the benefits and risks associated with hormone substitution. These facts have been largely related and commented in the general press. In this article, we review the important studies concerning this topic.