Radiation proctitis is a disease caused by irradiation of the rectum. Not all rectal irradiation causes radiation proctitis. Radiation therapy for rectal cancer at the standard doses and regimens does not usually cause proctitis; rather, proctitis occurs when other cancers are treated, such as those of the cervix, bladder, anus, and prostate. Severe radiation proctitis has become less common over the years as radiotherapy technology and techniques have improved. However, because radiation-induced disease does not improve over time (it’s the “gift that keeps on giving”), patients continue to present with proctitis following radiation that was given 30–40 years ago. In this chapter, a classification of radiation proctitis is described and its relevance to treatment is discussed.