Breast cancer is the most frequently diagnosed cancer in female worldwide and occurs as an interaction of genes and diet. As regards diet numerous studies all over the world have associated the disease with many foods and nutrients including soy and its compounds. Soy food and soy products are rich in phytoestrogens, naturally occurring hormone-like compounds with weak estrogenic effects. Despite inconsistencies in the available data, an inverse association between soy food consumption and breast cancer is likely. However, it seems that this correlation is more obvious in Asian rather than Western populations, where the consumption of soy is already higher. Moreover, the vast majority of studies that demonstrate this inverse association are case-control studies, a fact that should be taken into account. In this review, the current scientific evidence relating breast cancer and soy consumption is reported through a systematic way.