Microscopic colitis is an increasingly common cause of chronic diarrhea, and often causes abdominal pain and weight loss. The colonic mucosa appears normal or nearly normal endoscopically, and the diagnosis is made in the appropriate clinical setting when there is intraepithelial lymphocytosis and a mixed lamina propria inflammatory infiltrate. The 2 subtypes, collagenous and lymphocytic colitis, are similar clinically and histologically, and are distinguished by the presence or absence of a thickened subepithelial collagen band. Many potential pathophysiologic mechanisms have been proposed, but no convincing unifying mechanism has been identified. There are many anecdotal reports on treatment, but few controlled trials have been performed in these patients, although a systematic approach to therapy often leads to the satisfactory control of symptoms.