Left-sided diverticulosis coli is a common condition in western communities, with 30% to 50% of adults over the age of 60 being affected. It predominantly involves the sigmoid colon. The diverticula (pseudodiverticula) are pockets of mucosa bounded by muscularis mucosae and invested with a thin layer of submucosa, that are forced out through weak points in the muscularis propria, the tips ending in the colonic subserosa. The weak points in the muscle coat are the sites of entry of the nutrient vessels of the colonic mucosa. Diverticulosis is attributed to increased colonic intraluminal pressure while straining at stool in individuals who eat low-fiber diets. Muscular hypertrophy, shortening of the bowel, and thickened mucosal folds due to mucosal redundancy are characteristic of this condition. Complications of diverticulosis include bleeding, diverticulitis, peridiverticular abscess, perforation, stricture, and fistula formation. However, most individuals with diverticulosis are asymptomatic, without evidence of complications. Mucosal changes in the diverticula in uncomplicated diverticulosis include an increased lymphoid infiltrate, development of lymphoglandular complexes, mucin depletion, mild cryptitis, architectural distortion, Paneth cell metaplasia, and ulceration. The mucosa of the remainder of the sigmoid colon (ie, the nondiverticular mucosa) is usually normal, but in about 1% of cases it has features that are indistinguishable from ulcerative colitis or from Crohn's disease (segmental colitis associated with diverticular disease, SCAD). Such cases pose a difficult diagnostic challenge as patients with SCAD respond to medical or surgical therapy for diverticular disease, whereas those with ulcerative colitis or Crohn's disease will develop other manifestations of their disease in time and require different treatment. In SCAD, the mucosal changes are confined to the area of diverticulosis; therefore, histologic evaluation of the rectum (which is unaffected by diverticulosis) and more proximal bowel can be helpful in the differential diagnosis.