The pathomorphology of acute pancreatitis is reviewed in relation to etiology and pathogenesis. The common noninfectious acute pancreatitis which is usually caused by alcohol or gall stones starts with interstitial fat necrosis as the first visible and mildest lesion, and may end up with large hemorrhagic tissue necrosis as the most severe change. This sequence of alterations is called the type 1 necrosis pattern. Complications resulting from hemorrhagic necrosis are pseudocysts and abscesses. The infrequent type 2 necrosis pattern is characterized by initial duct necrosis and is found in noninfectious pancreatitis associated with prolonged circulatory failure in the course of a severe extrapancreatic disease. The also uncommon type 3 necrosis pattern refers to the changes found in many cases of infectious pancreatitis, which are characterized by initial acinar cell necrosis.