Most primary epithelial neoplasms found in the intrahepatic and extrahepatic biliary tracts and the pancreas have similar histologic appearances. The degree of similarity of tumor types is greater than is commonly appreciated and extends to rare neoplasms. It has been suggested that parallel classifications could be used for tumors arising in these sites. However, significant differences in terms used to denote histologically identical neoplasms are found in the current World Health Organization (WHO) classification of biliary tract tumors and the new WHO classification of exocrine pancreatic neoplasms. Mutations of oncogenes and abnormalities of tumor suppressor genes found in ductal adenocarcinomas from the two sites are also similar, although they have somewhat different frequencies. These histologic and molecular similarities provide strong support for the use of parallel descriptive diagnostic terminology for neoplasms arising in the pancreas and the biliary tract. The revised WHO classification of exocrine pancreatic neoplasms accommodates most types of biliary tract neoplasms and provides a new basis for standardization of nomenclature.