Acute sigmoid diverticulitis may present in different forms. Firstly, symptomatic diverticulosis and acute diverticulitis have to be differentiated to allow for the proper selection of treatment. In the initial phase of diagnostic procedures in addition to clinical features a few laboratory data, plain X-ray films of the abdomen and abdominal ultrasound are helpful to obtain information about the potential presence of perforation, abscess, tumor masses and the structure of the colonic wall. Computed tomography is capable to answer most of the questions in the diagnostic work-up of acute diverticulitis. On the other hand, contrast barium enema provides little additional information and should be considered as contraindicated in acute diverticulitis. If at all, water-soluble contrast media should be employed. Endoscopy has its place in the differential diagnostic evaluation of severe bleeding and suspected tumor, but should be reserved for these limited indications.