This review emphasizes the relationship of breast cancer, estrogen receptor and ligands, especially the centrality of the estrogen receptor, which mediates on one hand the hormone-induced gene transcription and on the other hand the anti-estrogen action against breast cancer. The characterization of the estrogen receptor ligand-binding domain co-crystallized with agonists or antagonists provided a molecular basis to gain an insight into the regulation of estrogen receptor and, thereby, to describe the mechanism of the hormone therapy in treating breast cancer.