Radiation therapy for breast cancer has gone through two revolutions in the last two decades: the routine use of radiation therapy in conjunction with breast-conserving surgery as an equivalent treatment to mastectomy, and the use of radiation therapy following mastectomy in advanced or node-positive disease. Indeed, the perception of postmastectomy radiation has gone full circle: from having no benefit when used for all cases, to being detrimental because of cardiac irradiation, to the present in which the selective use of irradiation in high-risk patients provides both an improvement in local control and an improvement of 8% to 10% in the survival rate. Improvements in radiation technique have reduced complications, in particular late cardiac deaths. The major issues still to be resolved are the targets for postmastectomy irradiation, determining which patients do not need radiation therapy for DCIS and for node-negative disease, and the efficacy of delivering radiation to just the affected quadrant rather than to the whole breast. At present, most patients approach radiation therapy for breast cancer with the knowledge that it has a very high probability of being successful.