Abstract : Radiotherapy is an important part of breast cancer treatment. After breast conserving surgery, a dose of 50 Grays (Gy) is administred to the entire breast. Boost by external radiotherapy or brachytherapy improves local control especially for women under fifty. For lobular in situ lesions, no additional treatment is required, while intraductal lesions are treated with post-operative radiotherapy in case of Van Nuys score of 7, 8 or 9. After mastectomy, irradiation is proposed in case of skin involvement, invasion of pectoral muscle, positivity of at least 3 axillary lymph nodes, SBR III grade, T3 stage or multifocality where the sum of tumour diameters are > 5 cm. Irradiation of the axilla has become exceptional. In node positive patients, the supraclavicular region receives 50 Gy with low energy photons, whereas the internal mammary chain is treated at the same dose but half is administred by low energy photons et half by electrons. Exclusive or preoperative irradiation is rare, because of new chemotherapy schedules, new drugs and hormonotherapy developments. Neoadjuvant chemotherapy enhances possibilities of conservative surgery, it is followed by radiotherapy depending on the same indications listed above. Re-irradiation is possible but one must be aware of the higher risk of late toxicity. Nowadays, major side-effects are rare but close follow- up during and after treatment is required from the radiation oncologist to detect, evaluate, prevent and even treat possible complications .