Intraepithelial lymphocytes interspersed between intestinal epithelial cells form a large population of T cells, mainly CD8+, close to the intestinal lumen. Together with lamina propria T cells and plasma cells, they represent the effector limb of the gut-associated lymphoid system. Recent data highlighting their original properties and their interactions with epithelial cells are reviewed; their contributions to immune defenses and gastrointestinal diseases in humans are discussed. Current Opinion in Gastroenterology 1993, 9:953-961