Irradiation has been used to treat cancer of the breast since 1895. Over the period between 1895 to the present, the therapeutic use of irradiation has evolved from a cauterizing agent to a sophisticated use of theory and practice to cause tumor cell death while at the same time producing maximum normal tissue sparing. The use of radiation has made it possible for the surgeon to use conservative breast sparing surgery to obtain excellent cosmetic results, while at the same time producing a local control equal to that of mastectomy. For the patient, conservative surgery becomes a better option than mastectomy because it not only produces local control equal to that of mastectomy, but gives the wonderful added benefit of a more superior and pleasing cosmetic result.