Prior to measles vaccine use, measles accounted for over 2.5 million deaths annually. Measles epidemiology in the developed countries is different from that in less developed countries. Whereas in the developing world, measles is a disease primarily of young children, particularly infants in urban areas, in the developed world, school-age children > 5 years old play a greater role. Prevention of measles in developing countries has relied principally on a single dose of Schwarz strain vaccine at age 9 months (> 85% efficacy); 80% coverage has prevented > 1.6 million deaths. However, problems have been encountered because of the narrow window to deliver vaccines between the time an infant becomes susceptible and exposure to disease. Recent studies suggest that some strains of measles vaccines given at potencies 10-100 times higher than standard vaccines may achieve good efficacy in infants aged 4-6 months, but safety of these vaccines has been questioned. Widespread use of standard vaccines in the West has resulted in dramatic reductions in measles incidence but has not prevented outbreaks among the 2-5% of persons not protected by a single dose. Such outbreaks often appear after extended periods either without measles or with low measles incidence. A single dose appears adequate to control measles well but inadequate to eliminate the disease. Many developed countries have adopted two-dose schedules. Measles immunization has dramatically reduced measles occurrence, but improved control is necessary to prevent the estimated 1 million deaths still occurring each year.