During 1946-2005, vaccine-preventable diseases were the topic of approximately 20% of all epidemic-assistance investigations by the Centers for Disease Control and Prevention. Both in the United States and abroad, current and former Epidemic Intelligence Service officers have played a critical role in describing the epidemiology of vaccine-preventable diseases, contributing to development of immunization policies, participating in the implementation of immunization programs, and establishing effective means for assessing adverse events following immunization. As newer vaccines are developed and introduced, they will continue to play similar roles and most likely will be involved increasingly in investigations of the factors that affect people's willingness to accept vaccination for themselves or their children.