Steroid hormone receptors (estrogen receptor [ER] and progesterone receptor [PR]) play a critical role in the development of breast cancer. Most importantly, the expression of ER and/or PR by tumor cells provides important information that is critical for the selection of treatment. Recent studies on ER and PR have provided new insights into the pathogenesis of breast cancer and the mechanisms of resistance to antihormonal therapy. Better understanding of steroid receptors, their ligands, and the mechanisms through which they exert their effects will allow the correct treatment to be targeted to responsive tumors.