Surveys conducted by the American College of Surgeons indicate an increasing trend toward the treatment of primary breast cancer by segmental mastectomy and radiotherapy. This operation has never been formally taught in training programs, and scant standard descriptive material exists. The National Surgical Adjuvant Breast Project conducted a series of workshops to develop guidelines for the performance of the operation. The essential points are (1) pathologic control of margins to ensure complete surgical excision, (2) surgical principles of complete tumor excision, (3) attention to optimum cosmetic results, and (4) axillary dissection for accurate staging and regional control of the disease. This article summarizes the recommendations made at those workshops.