Assuring quality in the practice of radiation oncology has become increasingly important because of the greater complexity of treatments and the advancing knowledge of the diseases that are treated. Increased public attention to errors in medicine, the increasing use of new imaging modalities to plan therapy, the availability of new therapeutic techniques such as intensity-modulated radiation therapy, and greater dependence on information technology have increased the need for intense quality assurance (QA) in the specialty. The process of QA and the resources available to radiation oncology staff members are described.