Computed Tomography is undeniably the most useful tool to confirm the suspected diagnosis of acute left-colonic diverticulitis and to objectively grade its severity into moderate diverticulitis (no signs of colonic perforation) and severe diverticulitis (signs of colonic perforation). Indeed, the severity of acute diverticulitis is statistically predictive of the risk both to need surgical treatment of the first episode of acute diverticulitis, and to follow a complicated evolution after successful conservative treatment of the acute phase. Consequently, CT brings a major contribution to define the place of surgery during the acute phase of diverticulitis, and, later on, inside the long-term evolution of the disease after initial successful conservative treatment.