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The American College of Radiology Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed every two years by a multidisciplinary expert panel. The guideline development and review include an extensive analysis of current medical literature from peer reviewed journals and the application of a well-established consensus methodology (modified Delphi) to rate the appropriateness of imaging and treatment procedures by the panel. In those instances where evidence is lacking or not definitive, expert opinion may be used to recommend imaging or treatment. The Atlanta Classification by the Acute Pancreatitis Classification Working Group recently modified the terminology for the clinical course and the morphologic changes identified on imaging, primarily contrast- enhanced multidetector computed tomography (MDCT). The two distinct clinical courses of the disease are classified as (1) early phase, which lasts approximately 1 week, and (2) late phase, which starts after the first week and can last for months after the initial episode. The two, primary, morphologic changes are acute, interstitial edematous and necrotizing pancreatitis. Timing of imaging, primarily MDCT, is based on the clinical phases and is, therefore, important for these imaging guidelines. Ultrasound's role is to detect gallstones after the first episode. MDCT plays a primary role in the management of acutely ill patients, only after a minimum of 48-72 hours and generally after one week. MR plays a supplementary role to MDCT. Follow-up MDCT guides management and therapy: percutaneous aspiration of fluid collections and/or placement of large caliber catheters in infected necrosis.