The rarity of small intestinal tumors and also the fact that they cause vague intestinal symptoms are both factors for a delayed diagnosis or an incidental diagnosis during surgery for other causes. Increased clinical suspicion and awareness of the factors that predispose to the development of such tumors are expected to increase the frequency of a timely diagnosis. This article focuses on the epidemiology, the histological typing, the predisposing diseases or precursor lesions and the prognosis of adenocarcinomas and endocrine (neuroendocrine) tumors that arise in the duodenum, the jejunum and the ileus. Information regarding the association between adenocarcinoma of the small intestine and familial polyposis coli, hereditary non-polyposis colorectal cancer, Crohn's disease, celiac disease, Peutz-jeghers syndrome and juvenile polyposis as well as evidence in favor of the adenoma-carcinoma sequence theory are given. Concerning the biologic behavior of the endocrine tumors of the small intestine which is notoriously difficult to predict, emphasis is given on the clinicopathological parameters which are important for prognosis.