Acute colonic diverticulitis (ACD) is an acute episode of severe and prolonged lower abdominal pain due to diverticular inflammation, usually associated with change in bowel movements, fever, and leukocytosis. Worldwide, computed tomography (CT) of the abdomen and pelvis with intravenous contrast is accepted as the best imaging method for evaluating the diverticular inflammation, serving the following functions: confirming the presence of ACD; evaluation of the disease severity and degree; therapy planning guide in presence of complications (such as abscess or intestinal perforation); diagnosis of other diseases that may simulate diverticular inflammation. In the literature, we found values of CT sensitivity for diverticular inflammation from 79% to 99%; CT is useful in differentiating other diseases, which may cause abdominal pain, when diverticular inflammation is not the cause, such as neoplasm, inflammatory bowel disease, appendix inflammations, epiploic appendix inflammation and colon ischemia. The trick to differentiate diverticulitis from other inflammatory diseases that involve the colon is the identification of diverticula in the pathological intestinal loop. In the last years, a radiological classification was created in order to guide the management of ACD in patients treated conservatively or with interventional procedures. The new classification system divides ACD into two groups: complicated and uncomplicated. Uncomplicated ACD is defined if only thickening of the intestinal wall is present, with increase of the perivisceral fat density. Complicated ACD is divided into 4 stages, depending on presence of microperforation without abscess and/or peritoneum involvement (stage 1 A), presence of abscess with diameter ≤4 cm (stage 1 B), presence of abscess with diameter >4 cm (stage 2 A), presence of distant air >5 cm from the pathological loop (stage 2 B), presence of diffuse fluid in at least two distant abdominal quadrants without distant free air (stage 3), presence of diffuse fluid and distant free air (stage 4). In this pictorial essay, we describe CT findings of the ACD and explain classification of the disease and its common and uncommon complications.