Radiation plays an important role in the management of breast cancer. The role of radiotherapy in the management of breast cancer can be defined in four categories as follows : ① primary radiotherapy in breast-conserving treatment(BCT) for early breast cancer including ductal carcinoma in situ(DCIS). ② Adjuvant radiotherapy after mastectomy for high-risk patients. ③ Radiotherapy with or without surgery after neo-adjuvant chemotherapy in locally advanced breast cancers. ④ Palliative radiotherapy for metastatic disease, mostly bone and brain metastases, and locoregional recurrences. Recently, the application of radiotherapy in the management of breast is increasing. The increased use of mammographic screening have dramatically increased the percentage of cases of breast cancer diagnosed at noninvasive or early stages of disease. As BCT has become the standard treatment for woman with DCIS and a preferred treatment method for early invasive cancers, the proportion of patients who need a radiotherapy is increasing. Also, post-mastectomy radiotherapy(PMRT) is re-appraised because some studies proved that PMRT not only decreased local recurrence but also improved survival rate. Many patients live for decades after treatment. Therefore, technical excellence in irradiating the intact breast or chest wall is very important to reduce long-term complications as well as to control the disease. Rationale, indications, and technical aspects of primary radiotherapy in breast-conserving therapy for early breast cancer and PMRT for operable breast cancer wareas reviewed.