Mixed endocrine-exocrine tumors of the gastrointestinal tract are rare neoplasms, which have been reported in the literature mainly as case reports and have been designated with a various and rather confusing terminology. In this review, on the basis of personally studied cases and of the analysis of cases reported in the literature, we have tried to identify types of mixed endocrine-exocrine tumors showing different clinicopathologic and biological characteristics. We have also tried to group the different clinicopathologic entities in prognostic classes which include: benign, low-grade, intermediate grade, and high-grade malignant mixed endocrine-exocrine tumors. The criteria for identifying the various types of mixed endocrine-exocrine tumors are extensively discussed.