Breast cancer continues to be the focus of intense basic and clinical research. In Valurne 1 of this series we dealt exclusively with topics concerned with therapy. In the present Valurne 2, we turn our attention to the experimental biology which is the foundation for our understand- ing of problems concerned with breast cancer etiology, mechanisms of hormone action, cell kinetics, experimental chemotherapy, and markers of tumor burden. The contributors to the volume are all noted scholars who are personally investigating these problems. The first chapter addresses the question, do hormones cause breast cancer? Segaloff provides us with a rational up-to-date overview of the existing data. He concludes that hormones by themselves are not tumor initiators but rather alter the hast environment so that other carcinogens are effective. lt is pointed out that the selection of the modeltest system is critical; one can almost assure any desired result by choosing an ap- propriately biased test system. The question of the role of viruses in the etiology of human breast cancer remains unanswered despite elegant studies in mause systems.