Transepithelial transport of antigens and pathogens is the first step in the induction of a mucosal immune response. In the intestine, the delivery of antigens across the epithelial barrier to the underlying lymphoid tissue is accomplished by M cells, a specialized epithelial cell type that occurs only in the lymphoid follicle-associated epithelium. Selective and efficient transport of antigen by M cells is considered an essential requirement for effective mucosal vaccines. Therefore, particulate antigen formulations are currently being developed to take advantage of the capacity of M cells to endocytose particles. Based on pathogens that exploit the M cell as an invasion route into the body, live mucosal vaccines have been designed using genetically-engineered, attenuated strains of pathogens such as poliovirus and Salmonella. In an alternative approach, antigens are coupled to or encapsulated in particulate synthetic carriers. To enhance binding and uptake of such nonviable vectors, ligands are being attached which direct the vaccine particle to receptors on the M cell surface.