This text reviews the latest ideas in tumor biology, explains the pharmacologic basis of the agents used, and discusses how they are applied in clinical practice. Part One presents an overview of the clinical importance of endocrine therapy, and describes the applications of the principal classes of drugs, including antiestrogens, aromatase inhibitors, progestins, and progesterone receptor antagonists. This section also discusses ovarian ablation. Part Two deals with the biological aspects of endocrine therapies, and relates current understanding of tumor biology and the pharmacologic basis of hormonal agents to their clinical applications. Among the topics covered are resistance and sensitivity to therapy. Part Three outlines future strategies, in particular the use of combination endocrine therapy, adjuvant growth factor therapy and the use of anti-invasive agents. This text is essential for oncologists and pharmacologists alike.