Aromatase inhibitors (AI) are a class of drugs used in the treatment of breast cancer and ovarian cancer in postmenopausal women. Aromatase is an enzyme that synthesises oestrogen and aromatase inhibitors block the synthesis of oestrogen, which is needed in some cancers to grow. As breast tissue is stimulated by estrogens, decreasing their production is a way of suppressing recurrence of the breast tumour tissue. In contrast to pre-menopausal women, in whom most of the oestrogen is produced in the ovaries, in post-menopausal women most of the body's oestrogen is produced in the adrenal gland from the conversion of androgens. Investigations are ongoing to look for other applications and researchers continue to study aromatase inhibitors as a way to stimulate ovulation or suppress oestrogen production in endometriosis. This book presents the latest research from around the world in this dynamic field.