Transient symptoms due to injury of the intestinal mucosa occur in the majority of patients receiving radiation therapy for pelvic or intra-abdominal neoplasms. Late (chronic) radiation enteropathy, although less common, is a more serious condition, associated with high morbidity and mortality. The manifestations of late radiation enteropathy are primarily due to changes in compartments other than the mucosa, such as intestinal wall fibrosis and obliterating vascular sclerosis. As a result of recent clinical and experimental studies, considerable knowledge about the pathogenesis, dose-response relationship, and time-course of development of late radiation enteropathy has been obtained. Also, the advent of new animal models has facilitated studies of time-dose-fractionation relationships in the intestine. The present paper summarizes clinical, pathophysiologic, and radiobiologic aspects pertinent to the development of chronic intestinal radiation injury.