Although the role of radiotherapy in breast cancer is controversial, there are clearly defined indications for its use. The probability of local relapse can be determined by careful evaluation of the histological features of the primary tumour, the absolute number of involved axillary nodes and the type and and extent of the surgery performed. Using these data, patients can be divided into a relatively low risk group who probably do not require radiotherapy and a group at significant risk of relapse who, even with adjuvant systemic therapy, are likely to benefit from postoperative radiotherapy. Despite the often-repeated view to the contrary, radiotherapy is more effective in the locoregional control of breast cancer if given postoperatively than if withheld until tumour relapse occurs. Radiotherapy at the time of relapse provides long-term control in only 40-55% of patients. This results in significant morbidity from uncontrolled local disease in the one-third of patients who survive 5 years and the one-quarter of patients who survive 10 years from the time of local relapse. This paper outlines the rationale and indications for the use of radiotherapy in improving locoregional control in breast cancer.