The history of infectious diseases together with the evidence of emerging and reemerging infections underscores the need for new vaccines and the obstacles to producing them. Vaccine adjuvants are usually thought of as agents that increase the intensity of immune responses and influence the balance between antibody and cell-mediated immunity at the cost of toxicity. This review uses copolymer adjuvants as examples to encourage a more sophisticated approach to the study of adjuvants as agents that can influence many parameters of immune responses including the specificity, titer, duration, memory, class, isotype, and avidity of antibody as well as the type of cell-mediated immunity and the incidence of genetic non-responders. Much research will be needed for design of effective vaccines against emerging and reemerging infections and for improvement of existing vaccines.