The clinicopathologic features and problems in classification and diagnosis of the pancreatic neoplasms with abundant mucus production are presented. In this article, the various reported concepts and terminology of these mucus-producing pancreatic tumors are summarized, and the differences between intraductal papillary-mucinous tumors and mucinous cystic tumors are specifically discussed. Intraductal papillary-mucinous tumors show diffuse or segmental dilatation of the pancreatic ducts with intraductal papillary growth. Mucinous cystic tumors are mucus-producing tumors showing cyst formation, which is often accompanied by intracystic papillary projections and "ovarian-type" stroma. Intraductal papillary-mucinous tumors occur most often in the pancreatic head of elderly men, whereas mucinous cystic tumors typically occur in the pancreatic tail or body of middle-aged women. Histologically, these tumors show a wide cytologic spectrum from benign to borderline to malignant. These tumors pursue an indolent clinical course compared with conventional ductal carcinoma of the pancreas. Mucinous cystadenocarcinomas have a higher malignant potential than intraductal papillary-mucinous adenocarcinomas, yet these tumors recur infrequently if they are excised completely. Because of the differences in clinicopathologic features, these tumors should be clearly separated from conventional ductal carcinoma of the pancreas.