Intraductal papillary mucinous neoplasms (IPMNs) and intraductal oncocytic papillary neoplasms (IOPNs) are the 2 types of intraductal neoplasms of the pancreas that may appear cystic because of dilatation of the ducts. Both are characterized by intraductal proliferation of mucinous cells usually arranged in papillary patterns. This proliferation is often associated with intraluminal mucin accumulation, which produces cystic dilatation of the ducts, mimicking mucinous cystic neoplasms. Endoscopic and radiologic studies and careful macroscopic examination are crucial for the correct diagnosis of IPMNs and IOPNs by showing the origin within the native ducts. Microscopically, these tumors display a spectrum of cytoarchitectural atypia that ranges from adenoma to borderline and to carcinoma-in-situ. Although they are defined as "intraductal tumors," IPMNs and IOPNs are associated with invasive carcinoma in about a third of the cases. It, therefore, appears that, like mucinous cystic neoplasms or pancreatic intraepithelial neoplasia involving the smaller ducts associated with ordinary ductal adenocarcinomas, these tumors are precursors of invasive carcinoma. Invasive carcinomas associated with IPMNs are of either tubular or colloid (mucinous noncystic) types, whereas those associated with IOPNs may be oncocytic. Even in the presence of invasive carcinoma, these tumors may follow a more protracted clinical course than ordinary ductal adenocarcinoma. On the other hand, rare examples of IPMNs after an aggressive clinical course despite the lack of any identifiable invasive carcinoma are on record. Therefore, IPMNs and IOPNs should be examined carefully and sampled extensively, first, to confirm that the main pathology is an intraductal process and, more importantly, to rule out the presence of an invasive carcinoma.