Although the beneficial effect of postoperative radiotherapy for breast cancer is well documented, this treatment may be related to a number of complications, which may affect patient quality of life and possibly survival. Among significant long-term irradiation sequelae are cardiac and lung damage, lymphoedema, brachial plexopathy, impaired shoulder mobility and second malignancies. The risk of these complications, particularly high with old, suboptimal irradiation techniques, has decreased with the introduction of modern technologies. In this paper, we review the contemporary knowledge on the toxicity of breast-cancer radiotherapy and discuss possible preventive measures.