The incidences of diphtheria and tetanus have been markedly reduced by the worldwide use of DTP vaccines. In Korea, tetanus outbreaks have disappeared since 1980s, and there has been no report on a patient with diphtheria since 1987. However, tetanus is still one of major health problems in many developing countries, and several serosurveillance studies in developed countries revealed that a substantial proportion of adult population may lack immunity against tetanus, and the immunity level against tetanus is continuously decreasing by age. In fact, few patients with tetanus have been reported since 1990s, and the immunity to tetanus is dramatically decreased in age groups over 30 years in our country. Also, persistent outbreaks of diphtheria in developing and developed countries have been reported. Many studies indicate that diphtheria outbreaks may occur under circumstances of ineffective immunization. In Korea, a few studies have reported that the diphtheria antibody titers in the 20~50 year age groups sharply decreased. Many studies confirmed that vaccine-induced immunity against tetanus and diphtheria did not last throughout the life, and serosurveillance studies suggested the existence of a large pool of susceptible individuals with a potential for epidemic infection. These results indicate that the Td vaccination program in adolescents and adults should be considered for maintenance of tetanus and diphtheria immunity.