Immunization registries are confidential, population-based, computerized information systems that contain data about children's immunizations and have been described as the cornerstone of immunization delivery in the 21st century. Work to ensure the privacy of registry participants and the confidentiality of their information, recruit provider participation, overcome technical and operational challenges, and identify sustainable funding streams has resulted in 24 percent of children less than 6 years of age currently in an immunization registry in the United States. New solutions will be needed before reaching the national health objective of increasing the proportion of children to 95 percent in a fully operational immunization registry by 2010.