M cells are distinctive epithelial cells that occur only in thefollicle-associated epithelia that overlie organized mucosa-associated lymphoidtissues. They are structurally and functionally specialized for transepithelialtransport, delivering foreign antigens and microorganisms to organized lymphoidtissues within the mucosae of the small and large intestines, tonsils andadenoids, and airways. M cell transport is a double-edged sword: Certainpathogens exploit the features of M cells that are intended to promote uptakefor the purpose of immunological sampling. Eludication of the moleculararchitecture of M cell apical surfaces is important for understanding thestrategies that pathogens use to exploit this pathway and for utilizing M celltransport for delivery of vaccines to the mucosal immune system. This articlereviews the functional and biochemical features that distinguish M cells fromother intestinal cell types. In addition it synthesizes the availableinformation on development and differentiation of organized lymphoid tissuesand the specialized epithelium associated with these immune inductivesites.