Breast cancer has become one of the most frequent malignancies among Korean women and the role of radiation therapy (RT) is expected to increase continuously. Current concepts in the treatment of breast cancer include a less radical surgery and the emergence of RT and systemic drug treatment. A surgery directed to minimize breast loss and RT to the whole breast has been the treatment of choice for early lesions. The use of whole breast irradiation (WBI) after the minimal breast surgery has been shown to significantly reduce the risk of recurrence in the affected breast and to increase the likelihood of long-term survival. Recently, there has been growing interest in the use of accelerated partial breast irradiation (APBI) as an alternative to WBI. APBI offers decreased overall treatment time and several advantages over WBI, including a decrease in the radiation dose delivered to uninvolved breast and adjacent organs. For the radiotherapy of breast cancer virtual simulation using the CT for three dimensional conformal RT has been widely used for external radiotherapy. The interstitial brachytherapy (insertion of radioactive source into the breast), balloon brachytherapy, and intraoperative electron therapy has been used for the APBI. Three dimensional radiotherapy including intensity modulated radiotherapy will be more widely used for breast cancer. RT fields are progressively reduced and APBI will be a realistic alternative in the future. Conclusion: The role of RT for treatment of breast cancer is important and the implementation of new RT technology will increase.