The chapters in this book are those presented at the Second International Symposium on Fundamental Problems in Breast Cancer held at Banff, Alberta, April 26-29, 1986. This is, therefore, the second volume in the series of symposia held in the Canadian Rockies. The aim of these symposia is to provide a supportive atmosphere for the development of new concepts as well as for the presentation of high quality scientific data. This is reflected in the chapters presented here. Many of the chapters put forward new hypotheses which will be tested in the clinic or laboratory in the next few years. In choosing the subjects for discourse, we preferred to tackle areas where controversy existed, and in this book we have grouped the chapters around discussion themes. The relative merits of mammography and physical examination in the detection of small lesions within the breast and the natural history of these lesions were important topics at the symposium, and the results of early detection studies will have a major effect on health care costs. We have devoted a large section of this book to this subject. The mechanism of growth and response of endocrine sensitive tumors and current concepts of the statistics and cost-benefit assessment of adjuvant chemotherapy trials comprise two other sections of the book where original and stimulating ideas were presented.