Critical misconceptions about vaccine development have arisen in the context of acquired immunodeficiency syndrome (AIDS) vaccine research. These include: the goal of vaccination; the biological relevance and predictive value of animal models; the meaning of "correlates of protective immunity"; the nature and duration of vaccine-induced immune responses; and the need for multiple, iterative field trials. In this article, lessons from the history of successful vaccine development relevant to these issues are discussed. Clarity about these central issues and adherence to a common vocabulary are important for the process of establishing an appropriate, milestone-driven process for developing safe, effective AIDS vaccines.