According to the Atlanta classification, the most widely accepted clinically based classification system for acute pancreatitis, four pathologic entities of fluid collections and necrosis are recognized. Acute fluid collections occur early as an exudative reaction to the pancreatic inflammation, have no fibrous wall, and resolve spontaneously. Pancreatic necrosis, the most severe form of acute pancreatitis, is diagnosed on dynamic contrast-enhanced computerized tomography and requires early aggressive cardiorespiratory resuscitation, nutritional support, and appropriate systemic antibiotics to prevent superinfection. Development of infection (infected necrosis) is the indication for operative debridement, preferably as late in the course of the disease as possible. Acute pseudocysts are collections of pancreatic, enzyme-rich fluid caused by pancreatic ductal disruption that occur 3 to 6 weeks after onset of acute pancreatitis and have a well-defined, nonepithelial fibrous wall. If communication with the ductal system is present, internal enteric drainage (either operative or endoscopic) is more effective; if communication is not present, the pseudocysts are amenable to percutaneous drainage. A pancreatic abscess is an infected, circumscribed peripancreatic collection, associated with minimal or no parenchymal necrosis, that occurs late (4 to 6 weeks) after onset of severe pancreatitis and may represent an infected pseudocyst; percutaneous drainage is the treatment of choice.