Rapid developments in imaging techniques, including functional imaging, have fueled the drive to implement new methods of delivering three-dimensional radiation therapy, such as intensity-modulated radiation therapy (IMRT), with unprecedented accuracy. In comparison with three-dimensional conformal radiation therapy (3D-CRT) it is now possible with IMRT to escalate the absorbed dose in the target volume for the same normal tissue dose and/or to reduce the normal tissue dose for the same tumor dose, resulting in improved tumor control and/or less normal tissue complications. IMRT is accomplished by the sequential isocentric delivery of multiple small beams typically of non-uniform intensity that can lead to the generation of very steep dose gradients. To meet the requirements of such sophisticated beam delivery techniques both tumor and normal tissue volumes must be delineated in 3D with high precision. The use of dose-volume histograms (DVHs) for the specification of absorbed dose is inherent to the treatment modality. The demands on treatment planning systems to ensure accurate dose delivery necessitate the employment of complex iterative optimization processes. The stringent requirements on precision for the entire treatment process from imaging to treatment entail the implementation of comprehensive commissioning and quality assurance programs. The present report provides the information necessary to standardize techniques and procedures and to harmonize the prescribing, recording, and reporting of IMRT where possible with those of other modalities. Applicable concepts and recommendations in other ICRU reports concerning radiation therapy are adopted and extended where required. Clinical examples of IMRT are provided to illustrate the recommendations of prescribing, recording, and reporting that are contained in this report. As the present report describes in some detail the physical, technical, treatment planning and clinical aspects of IMRT, it should be a useful reference for current practitioners and should also provide new and potential users, as well as other interested readers, with the basic background to enable them to understand the techniques involved and the requirements for implementing IMRT.