The EBCTCG has clearly demonstrated that the use of RT after either breast conserving surgery (BCS) or after mastectomy in node-positive patients not only reduces local recurrence (LR), but also improves long-term survival. The EBCTCG specifically found that the absolute reduction in the 5-year rate of LR was proportional to the absolute reduction in 15-year breast cancer mortality with a 4:1 ratio. Studies from the EBCTCG have also clearly shown that when RT for breast cancer inadvertently delivers 'excessive' dose to the heart, there is an increased rate of late cardiac deaths. Over time, LR rates with RT have decreased, particularly after BCS, and this is largely due to a favorable interaction of RT with adjuvant systemic therapy. There is new information emerging about the effectiveness of RT based on newer biologic classification of breast cancer and about how the 4:1 ratio might change in the face of increasingly effective systemic therapy and with a better understanding of the importance of biologic classification and of the competing risks of local and distant recurrence. Technical innovations in RT include the development of techniques that minimize cardiac irradiation. It is critical that CT simulation be done during which the heart should be contoured and the radiation dose-cardiac volume relationship be determined. In the EBCTCG's preliminary analysis, increased late cardiac deaths were most closely linked to mean cardiac doses >5 Gy. A number of techniques are available to minimize cardiac dose both after BCS and mastectomy and these will be described. There has also been development of fractionation and treatment techniques to complete RT faster for purposes of patient convenience and to assure its optimal level of use. There are now 10-year results from a Canadian trial showing equivalent LR and cosmetic outcome with RT using the conventional 25 treatments to the whole breast compared to 16 treatments using a higher dose per day. Few randomized patients, however, were treated with adjuvant chemotherapy and a boost was not used. The generalizability of the Canadian trial results to patients treated with adjuvant chemotherapy or requiring a boost is not known. For patients treated after BCS, there has been great interest in the use of accelerated partial breast irradiation (APBI), which can be accomplished using interstitial, intracavitary, external-beam or intraoperative techniques. Clinical trials are now underway to compare APBI to conventional techniques, but mature results from these trials will not be available for some time. In the U.S., ASTRO has developed a consensus statement based on an expert panel as to when APBI can be reasonably used outside of a clinical trial and this will be described.