Since taking office, President Clinton has devoted considerable attention to childhood immunization and to the overall U.S. policy toward vaccine development, delivery, and pricing. But the reliability of U.S. vaccine supplies has received far less attention, despite several recent interruptions in vaccine production and supply. Moreover, the increasing producer concentration in the U.S. and global vaccine industries raises the possibility that more and more important vaccines will be produced by a single firm, often in a single production facility. Is a federal policy for vaccine supply assurance necessary, and what form should it take? Are sole-source suppliers a weak link in the U.S. vaccine supply chain? Are the recommendations of the Institute of Medicine's Committee on Emerging Infections for a publicly owned "standby production capacity" of critical vaccines feasible or cost-effective? We consider these and other issues in our discussion of the U.S. vaccine industry, the potential role of foreign vaccine suppliers, and the use of existing federal facilities and policies to address a possible interruption in critical vaccine supplies.