Radiation therapy has long been used in the treatment of breast cancer. However, it is still very difficult to state with certainty whether or not radiation therapy is truly beneficial for patients with breast cancer, mainly because breast cancers are very heterogeneous in their clinical behavior, and because radiation therapy has undergone significant change in the methodology. We extensively reviewed the literature, and determined that radiation therapy increases the survival of some patients, particularly in conjunction with surgery and systemic therapy. Although the total proportion of such patients seems to be small, appliedto the general population of breast cancer patients, the absolute number of women who might benefit from radiation therapy may be quite large.