A wide range of etiologies and pathogenic mechanisms underlie colitis. This Review provides an overview of the pathophysiology, epidemiology, histopathology, and clinical characteristics of noninfectious and non-IBD forms of colitis: microscopic colitis, Behçet's syndrome, diversion colitis, diverticular colitis, eosinophilic colitis, ischemic colitis, and radiation colitis. These more recently characterized and rare forms of colitis occur as either primary conditions or complications of other diseases. Most of these diseases are uncommon; therefore, epidemiologic data and data from controlled trials are not readily available. Practical guidelines for the diagnosis and therapy of these more recently characterized and rarer forms of colitis are given where possible.