The control of breast cancer, a leading cause of cancer death in women, will depend ultimately on our understanding of the disease—its origin, and progression which in turn will permit the effective management of its treatment, its detection, and perhaps even its prevention. It is for a better understanding of this spectrum of biological processes crossing back and forth across scientific and clinical disciplines that this volume strives. Several broad topics have been addressed in organizing a large mass of work representing state of the art updates from many of the major breast cancer research groups around the world. The chapters in the first section speak to the factors affecting the growth and development of normal and malignant mammary epithelium. Special emphasis is placed on insights drawn from developmental biology, the cellular interactions that occur in the mammary gland during growth and differentiation; and the study of hormones and growth factors in the regulation of growth and differentiation of normal and malignant breast tissues. In the section on the biology of breast cancer, there is a characterization of relevant model systems for the study of breast cancer and their contribution to our understanding of preneoplasia and progression in mammary cancer. Included as well is the current status of major studies on the immunological aspects of breast cancer and the latest efforts in the development of markers for metastasis in breast cancer.