Peritonitis is defined as an inflammatory process of the peritoneum caused by any irritant/agent such as bacteria, fungi, virus, talc, drugs, granulomas, and foreign bodies. It may be infective or noninfective. It is a potentially life-threatening intra-abdominal pathology worldwide which partly manifests as an acute abdomen. The mortality of peritonitis in the early 1900s was close to 90%: with the introduction of antibiotics, the mortality is nowadays strongly decreased. Among acute abdominal pain in the elderly patients, common etiologies of peritonitis include appendicitis, diverticulitis, perforated gastroduodenal ulcer, perforated carcinoma, acute cholecystitis, acute pancreatitis, and abdominal surgery. Peritonitis in elderly patients is quite different compared with infections in younger patients owing to immunosenescence, epidemiological and bacteriological aspects, and associated comorbidities. Signs and symptoms are often atypical, leading to supplementary investigations that often delay the diagnosis and appropriate treatment and which could place the patient at a higher risk of death. Diagnostic imaging is essential for diagnosing and managing a variety of patient presentations. Frequently in the emergency department, imaging is required and a rapid interpretation is demanded to aid in the acute management of a critically ill patient. Multidetector row computed tomography is currently the primary imaging examination for evaluating elderly patients with an acute abdomen and is well suited for diagnosing and characterizing the most important causes of acute abdominal pain. On multidetector row computed tomography, normal peritoneum appears as a fine, thin structure, and therefore it is hardly detectable. The pathologic involvement produces a thickening of the peritoneal layers, which become easily noticeable.