Pancreatic intraductal papillary mucinous neoplasms (IPMNs) are mucin producing neoplasms with frequent papillary architecture that arise within the pancreatic ducts and are increasingly being recognized. Because they exhibit a spectrum of dysplasia ranging from low grade to high grade and may also have associated invasive carcinoma, and because they are clinically detectable, they are now intensively studied. There is marked overlap between IPMNs and pancreatic intraepithelial neoplasia (PanIN), such that the distinction between these two lesions is nearly impossible in certain cases. In addition, IPMNs sometimes can be confused with other primary cystic lesions of the pancreas. As a result, the correct diagnosis of IPMN can be challenging. This review addresses the clinical and pathological features of IPMNs, emphasizing their diagnostic criteria, differential diagnosis, and biologic behavior. Problematic issues in the pathologic evaluation of IPMNs are discussed.