The concept that hormones influence tumor growth originated in 1889 with the proposal of Albert Schinzinger who suggested that breast cancer is related to the ovaries. Several years later, Sir George Beatson observed that remission of disseminated breast cancer could be achieved in premenopausal patients by performing bilateral oophorectomy. As a result of the contri­ butions of Hedley Atkins, Charles Huggins and others, additive and ablative hormonal therapies have been widely used for the treatment of advanced breast cancers for several decades. Model systems to study the effects of hormones on growth and regression of breast tumors have been available for many years; however, the complexities of the hormonal environment have rendered in vivo studies difficult in man and experimental animals. Recently, the availability of long-term cultures of breast cancer cells has stimulated many investigators to use these cell lines to unravel the mechanisms of hormone action. Because of the extreme diversity and complexity of advances regarding the endocrinology of the breast and breast cancers, a multi-authored review was deemed necessary. It has been gratifying to receive contributions from many noted scholars. In Volume I of this monograph, the influence of steroid hormones and their antagonists upon normal and neoplastic tissues of the mammary gland are presented. In Volume II, the effects of peptide and other hormones are reviewed.