Recent data on phenotype, differentiation, and functions of intestinal intraepithelial lymphocytes (IEL) in rodents and in humans are reviewed in this article. IEL form a large population of activated T cells located at the interface between the body and the septic intestinal content. Phenotypic studies indicate that IEL are mainly CD8-positive cells that contain both TCR alpha beta and TCR gamma delta cells in variable proportion depending on the species and antigenic intraluminal stimulation. Studies in experimental models suggest a dual origin, both thymus-dependent and thymus-independent of IEL, and a role of the gut epithelium in the differentiation of IEL. Functional studies allow insight on the mechanisms by which IEL can protect the intestinal epithelium or conversely, when abnormally activated, induce epithelial damage.