Overall, the diagnosis of diverticulitis is more reliably made by computed tomography (CT) than by ultrasound (US). However, since US is often used as a first modality in acute abdomen, it is important to be aware of the US signs of diverticulitis. Besides, in not too obese patients, US may be superior to CT. US is most useful in early, uncomplicated diverticulitis. Daily, repeated US examinations in patients with diverticulitis have taught that diverticulitis, in the majority of cases, runs a predictable and benign course. Initially, there is local wall thickening of the colon with preservation of the US layer structure. Within the inflamed diverticulum, a fecolith is present, and the diverticulum is surrounded by hyperechoic, noncompressible tissue, which represents the inflamed mesentery and omentum ‘sealing off’ the imminent perforation. US follow-up shows evacuation of the fecolith to the colonic lumen, with or without the transient development of a small paracolic abscess, sometimes with disintegration of the fecolith. This process of spontaneous evacuation of pus and fecolith via local weakening of the colonic wall at the level of the original diverticular neck towards the colonic lumen takes place within 1 or 2 days, rarely longer. The residual inflammatory changes remain present for several days after the evacuation, and it is not uncommon to find an empty diverticulum at first presentation. If, in such cases, patients are specifically asked for their symptoms, they invariably declare that ‘the worst pain is over’. Whenever diverticulitis takes a complicated course, CT is superior to US, especially in the detection of free air, fecal peritonitis and deeply located abscesses, and in general in obese patients. Finally, US, if necessary followed by CT, has an important role in the diagnosis of alternative conditions: ureterolithiasis, pyelonephritis, perforated peptic ulcer, appendicitis, Crohn’s disease, epiploic appendagitis, gynecological conditions, colonic malignancy, pancreatitis, etc. Right-sided colonic diverticulitis in many respects differs from its left-sided cousin. Diverticula of the right colon are usually congenital, solitary, true diverticula containing all bowel wall layers. The fecoliths within these diverticula are larger and the diverticular neck is wider. There is no hypertrophy of the muscularis of the right colonic wall. My observations with US and CT in 110 patients with right colonic diverticulitis clearly show that it invariably has a favorable course and never leads to free perforation or large abscesses. Although relatively rare (left:right = 15:1), it is crucial to make a correct diagnosis since the clinical symptoms of acute right lower quadrant pain may lead to an unnecessary appendectomy or even right hemicolectomy.