Most abdominal hernias can be diagnosed on the basis of findings on physical examination or plain films and barium studies. However, diagnostic dilemmas can arise when patients are obese or have had surgery. Cross-sectional CT scans can show hernias and the contents of the peritoneal sac. More important, CT findings can be used to diagnose unsuspected hernias and to distinguish hernias from masses of the abdominal wall, such as tumors, hematomas, abscesses, undescended testes, and aneurysms. This essay illustrates the CT findings in diaphragmatic hernias, internal hernias, and abdominal wall hernias.