Patients with signs of an acute abdomen continue to be a challenge for both the emergency physician and the intensivist. Clinical symptoms usually result from secondary peritonitis possibly progressing to intraabdominal sepsis. Critically ill patients need rapid diagnostic work-up and an interdisciplinary therapeutic approach. Among patients with secondary peritonitis, those with postoperative peritonitis (e.g., after anastomotic leakage) show a particularly high mortality because of unspecific symptoms. Beyond routine diagnostic procedures, patients with an acute abdomen often require a CT scan which helps to detect the septic focus, thereby often allowing an interventional source control. Therapy consists of three main elements: source control, broad-spectrum antimicrobial therapy, and supportive intensive care medicine.