The mucosae represent the primary point of contact between the major respiratory and enteric pathogens and the innate and adaptive immune response. The microanatomy and function of the mucosal immune system is now well-characterized, in particular the major effector mechanism that involves the production and translocation of secretory immunoglobulin A (IgA). Mucosal delivery of antigen has the potential to induce potent local and systemic immunity, although such responses may differ between neonatal, infant and adult mice. In younger animals mucosal immune responses are Th2 biased, whereas in adults there is a broader Th1 and Th2 responsiveness. There is much interest in the development of mucosally delivered vaccines which can be tailored to enhance Th1 immunity or to avoid potential interference from maternally derived antibodies (MDA). Accordingly, a range of mucosal adjuvants (particularly those derived from bacteria) has been tested, and live recombinant vectored vaccines may also be effectively delivered by this route.