Recent efforts to attain near-complete coverage of child populations by recommended vaccines have included initiatives by federal and state agencies, as well as private foundations, to develop and implement statewide or community-based childhood immunization registries. Plans for a single, national registry have been set aside in favor of a national network of local and state registries linked through the use of common definitions and unique child identifiers. However, both operational/technical and financing difficulties have slowed their development. The experience to date in selected areas has provided useful lessons for further development of a registry system and has underscored the potential of such systems to assure the success of childhood immunization initiatives.