Intraoperative radiotherapy (IORT) is a technique where a high, single-fraction radiation dose is delivered during a surgical procedure to macroscopic tumours or tumour beds with minimal exposure of surroundings tissues which are displaced and shielded during the procedure. In this paper, the rationale for and use of IORT, both with electron beams (IOERT) and high-dose-rate brachytherapy (HDR-IORT) are discussed. For most tumours, the likelihood of obtaining local control (LC) improves when increasing doses can be administered. In many clinical situations, however, the dose that can be delivered safely to the tumour target is limited by the risk of damaging normal tissues. Special consideration is therefore given on this paper to the relationship between dose, LC and possible complications. Criteria for patient's selection and evaluation and information on sequencing and techniques are presented as well as some considerations on the need for a proper programme on quality assurance and periodical reporting of data.