Viral vaccines have been successfully administered by the mucosal route for two decades now. However, only in the last 10 years have the concepts involved in mucosal immunocompetence been well characterized. It is apparent that the bronchus-associated lymphoid tissue, the gut-associated lymphoid tissue, and the immunocompetent cells present in ocular tissue, conjunctiva, nasopharynx, genital tract, and secretions of the mammary glands represent the components of the common mucosal immune system. The predominant immunologic activity in the external mucosal surfaces is associated with secretory IgA and T lymphocytes, with variable contributions from other immunoglobulins and macrophages. Evidence presented in this review suggests that immunization via the mucosal routes of the respiratory and gastrointestinal tracts is the most effective means of inducing effective immunity in the mucosal surfaces as well as in the systemic tissues. Available experience with mucosal immunization with poliovirus, rubella virus, measles virus, and adenovirus vaccines in humans and with other viral vaccines in animals is briefly reviewed.