Because microparticles are taken up across the gastrointestinal tract following oral administration, they may be exploited for the oral delivery of labile compounds. For example, microparticles have been used for the oral delivery of peptide and protein drugs, and have resulted in improvements in bioavailability. In addition, microparticles have also been exploited for the oral delivery of vaccines, inducing potent immune responses and protective immunity. However, the extent of uptake of microparticles across the gut may limit their potential for oral delivery. Therefore, intranasal immunization is an attractive approach for the induction of mucosal immunity. Microparticles have also been used for the delivery of vaccines to the respiratory tract. In addition to the use of small microparticles to target antigens to mucosal lymphoid tissues, polymeric coatings may also be employed simply to protect antigens against degradation during transit in the gut. Hence, oral delivery using polymers is not necessarily dependent on the uptake of the delivery system across the gut. Recent studies have indicated that a number of polymeric delivery systems possess significant potential for the development of mucosally administered vaccines. However, further work is needed in a number of areas, including the stabilization of antigens within the polymeric delivery systems.