for a human adult. These surfaces are covered by thin epi- thelial layers that are protected from potentially harmful microorganisms by innate and adaptive defence mecha- nisms. Intestinal mucosal surfaces are in continuous contact with a heterogeneous population of microorgan- isms of the endogenous flora (up to 10 11-12 per g of lumenal material in the colon) and are exposed to food and microbes. A major role for the mucosal epithelia is in barrier function, essential for preventing colonization or invasion of the host by foreign microorganisms. Epithelial tissues also provide the mucosal immune system with a continuous stream of information about the external envi- ronment. Depending on the nature and the dose of the antigens transported from the gut lumen into mucosal lym- phoid tissues, strong immune responses or unresponsive- ness can be induced. Immune responses participate in the elimination of pathogenic microorganisms, while immune tolerance prevents harmful reactions against the gut flora and food antigens. The type of immune response triggered by environmental antigens appears to depend, in part, on initial recognition by the innate immune system. Recent data underscore the importance of innate immunity in sensing the microbial environment and the role of the epithelium in releasing signals that allow recruitment of pro-inflammatory leucocytes, immune cells, or both. We shall review these various aspects of mucosal immunity, with special emphasis on the cross talk that takes place between the microflora, the epithelium and the immune cells in the gut.