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Despite the use of measles vaccine for over 30 years, measles continues to occur even in highly vaccinated populations. The goal set by the Pan American Health Organization to eliminate measles from the Americas has not been accomplished yet. Although live-measles attenuated vaccines have dramatically reduced the incidence of measles infection, relatively little is known about the immune response generated by vaccination. The principal objective of the work described in this thesis was to perform detailed analysis of cellular and humoral responses to primary measles vaccination in children from the developed and developing worlds. Studies involving Canadian children ranging in age from 12 months to 15 years of age demonstrated that cellular "memory" for measles antigens (lymphoproliferation) was induced in only 50--60% of vaccinees but that, in some children, this cellular response was more durable than anti measles antibody production. Phenotypic studies in these children demonstrated an evolution of early CD8+ T cell activation with later CD4+ T cell activation over a 5--8 week period after vaccination. Expression of CD30, a putative Th2 marker and the costimulatory molecule CTLA-4 on CD4+ and CD8+ T cells, was associated with a strong humoral response while increased production of IFN-gamma and IL-12 was associated with a strong LP response to vaccination. Similar, though less extensive, studies in Peru revealed that less than 25% of these developing world children mount detectable measles-specific LP responses after vaccination, despite having high antibody titers. Although there are many differences between the two study populations (e.g.: race, age at vaccination, vaccine strain), one especially striking difference was the relative "maturity" of T cells in Peruvian children (CD45RO expression) and the marked degree of PBMC activation at the time of vaccination in this setting.