Radiotherapy is an essential part of the multimodality treatment of breast cancer. Applying safe and effective treatment requires appropriate facilities, staff, and equipment, as well as support systems, initiation of treatment without undue delay, geographic accessibility, and completion of radiotherapy without undue prolongation of the overall treatment time. Radiotherapy can be delivered with a cobalt-60 unit or a linear accelerator (linac). In early stage breast cancer, radiotherapy is an integral part of breast-conserving treatment. Standard treatment includes irradiation of the entire breast for several weeks, followed by a boost to the tumor bed in women age 50 years or younger or those with close surgical margins. Mastectomy is an appropriate treatment for many patients. Postmastectomy irradiation with proper techniques substantially decreases local recurrences and improves survival in patients with positive axillary lymph nodes. It is also considered for patients with negative nodes if they have multiple adverse features such as a primary tumor larger than 2 cm, unsatisfactory surgical margins, and lymphovascular invasion. Many patients present with locally advanced or inoperable breast cancer. Their initial treatment is by systemic therapy; after responding to systemic therapy, most will require a modified radical mastectomy followed by radiotherapy. For those patients in whom mastectomy is still not possible after initial systemic therapy, breast and regional irradiation is given, followed whenever possible by mastectomy. For patients with distant metastases, irradiation may provide relief of symptoms such as pain, bleeding, ulceration, and lymphedema. A single fraction of irradiation can effectively relieve pain from bone metastases. Radiotherapy is also effective in the palliation of symptoms secondary to metastases in the brain, lungs, and other sites. Radiotherapy is important in the treatment of women with breast cancer of all stages. In developing countries, it is required for almost all women with the disease and should therefore be available.