There is now general agreement that treatment with conservative surgery and radiation therapy yields survival equal to mastectomy with the advantage of organ preservation for properly selected patients. When competently performed, such treatment gives highly satisfactory cosmetic results and acceptably low rates of local tumor recurrence. However, there remain numerous controversies concerning patient selection for this treatment option. The factors involved in patient selection may be grouped into three categories: patient factors; clinical factors; and pathologic factors. This article reviews their use. Because breast cancer has a long natural history, long follow up of patients is required for ultimate proof of the relative merits of different selection or treatment policies. However, due to the increasing numbers of patients being treated with conservative surgery and radiotherapy, it appears likely that many of these questions will be answered within the next decade.