Postoperative radiotherapy (RT) is an important part of the multidisciplinary treatment of early breast cancer. Meta-analyses of large randomized trials of RT after lumpectomy and mastectomy have shown that RT reduces the risk of local recurrence with at least 70% and mortality after 15 years with 5%. The effect is most pronounced in patients with a high risk of local recurrence, but is independent of adjuvant systemic treatment. Late radiation effects, in particular cardiac mortality can now be avoided using modern treatment technique. However, there is a continuous need to find the overall balance between the risk and benefit of RT based on individual prognostic parameters and technical possibilities.