Hormonal therapy, such as estrogen-targeting therapy, has undergone remarkable development in recent several years, using drugs such as LH-RH agonists, new SERMs and third-generation aromatase inhibitors. Several ongoing large-scale international clinical trials for hormonal therapy are establishing the standard protocol for treatments with these drugs. On the other hand, there have been attempts to predict the individual efficacy of hormonal therapy using classical molecular biomarkers such as ER and PgR. However, approximately one-third of ERalpha-positive patients do not respond to endocrine therapy, while some ERalpha-negative patients are responsive. These discrepancies may be due to the different estrogen-related intracellular signaling pathways in breast cancer cells. Furthermore, the ineffectiveness of hormonal therapy in some individuals (due to, for example, aromatase inhibitor resistance) may be caused by these mechanisms. In this paper, we discuss the molecular mechanisms of these different responses to hormonal therapies and their implications for the estrogen signaling pathway in breast cancer cells. Furthermore, we touch upon basic studies into predicting the efficacy of hormonal therapy and new strategies in this field.