The idea for this book arose during the 1985 Gordon Conference on "Mammary Gland Biology." New developments in the methodology of cell biology and the explosive growth of molecular biology had begun to impact upon our understanding of mammary gland growth and function. It seemed a propitious time for summarizing the current status of knowledge of the cell and molecular biology of mammary cancer and for attempting to outline future areas of concern and interest. The reviews presented here were completed by the Fall of 1986. Although new insights will surely continue to emerge, it is hoped that the material in this volume will form not only a current update but a basic core of information for future experiments. We have not attempted to cover all areas of mammary gland transformation. Those areas where recent detailed reviews are already available have been omitted. Also, the areas of normal gland development, cell ultrastructure, hormone responsiveness, chemotherapy and clinical aspects of mammary cancer have not been included. Instead, we have selected those areas where the development of new methodology, reagents and results have led to new ideas about mammary gland function and development as they are related to neoplasia.