Breast cancer is the major cancer killer of women in the United States. Recent advances in therapy promise more effective control of this illness. The development of hormone receptor analysis permits us to better understand the mechanisms of action of steroid and peptide hormones and increases the likelihood of using there hormonal agents effectively. Prospective surgical and surgical adjuvant trials offer the opportunity to define accurately the appropriate surgical procedure and most effective postoperative therapy after initial surgery. The development of chemotherapy, first singly and now in combination, has led to improved treatment of patients with advanced disease and defined better chemotherapy for use in adjuvant settings. The investigation of a matrix of marker substances in breast cancer may uncover new methods to assess residual disease and to monitor disease recurrence.