Aromatase inhibitors (AI) are a class of drugs used in the treatment of breast cancer in post-menopausal women. Aromatase inhibitors work by inhibiting the action of the enzyme aromatase which converts androgens into estrogens by a process called aromatization. In contrast to pre-menopausal women, in whom most of the estrogen in produced in the ovaries, in post-menopausal women most of the body's estrogen is produced in the adrenal gland from the conversion of androgens. Because some breast cancers respond to estrogen, lowering the estrogen level in post-menopausal women using aromatase inhibitors has been proven to be effective in breast cancer treatment. Aromatase inhibitors are not useful in women who are pre-menopausal, however, because AIs do not inhibit ovarian estrogen production.

Investigations are ongoing to look for other applications. Increasingly, for example, researchers are using aromatase inhibitors to stimulate ovulation (in a manner similar to, but not exactly the same as, clomiphene citrate).