Since a radiotherapy department is a clinical and technical entity, quality assurance (QA) is a team effort. This is necessary for the treatment of a patients with the required high degree of accuracy and precision. The physical and technical aspects of QA programmes performed in radiotherapy institutions have been reviewed in this paper. Firstly, the accuracy required in radiotherapy has been discussed. An increasing amount of clinical evidence indicates that an accuracy of 3.5% in the dose value at the specification points and an accuracy of less than 5 mm in the position of the field with respect to the target volume in the patient is required. Secondly, various aspects of QA programmes related to beam characteristics of treatment machines, treatment planning and treatment verification have been elucidated. It is recommended that international organizations formulate minimum and ideal QA programmes for this purpose. Finally, some recent developments in the field of treatment verification concerning portal imaging and in vivo dosimetry, partly sponsored by the CEC/AIM programme, have been illustrated in more detail. The latter project concerns the further development of an on-line electronic portal imaging device for checking the correct position of the target volume with respect to the radiation beam.