 Endocrine blockade in breast cancer can be achieved by reducing the levels of estrogens through ovarian ablation medical, surgical, or through radiation in pre-menopausal women or with the administration of aromatase inhibitors in post-menopausal women. Direct inhibition of estrogen receptors can be achieved by administering selective estrogen receptor modulators, such as tamoxifen, or a pure estrogen receptor antagonist such as fulvestrant. In post-menopausal women with advanced breast cancer, current standard first-line endocrine therapy comprises of a reversible aromatase inhibitor such as letrozole or anastrozole. Other endocrine therapy options in advanced breast cancers include a irreversible aromatase inhibitor exemestane, tamoxifen, megestrol acetate, and more novel combinations of an aromatase inhibitor with fulvestrant combined estrogen blockade, exemestane combined with an mTOR inhibitor everolimus, or letrozole combined with a CDK46 inhibitor palbociclib. 