Breast conserving treatment is defined as a combination of conservative surgery for resection of the primary tumor (lumpectomy, segmental mastectomy, or quadrantectomy), mostly with dissection of the axillary nodes, followed by radiation therapy for the eradication of residual microscopic disease in the breast and, in some cases, draining nodal tissues. The goal of this treatment is to provide highly satisfactory cosmetic results, without compromising local tumor control or survival compared with total mastectomy. This review article summarizes the results of prospective randomized trials comparing breast conserving treatment with mastectomy, the NIH consensus conference statement, patient selection, complications, cosmetic outcomes, the present situation of this treatment in Japan and our clinical results using CT simulator for radiotherapy treatment planning.