Now celebrating its 26th year of existence, the Vietnam Era Twin Registry continues to be one of the largest national samples of adult twins in the United States. The Registry twin member population is composed of 7,369 US male-male twin pair Veterans (14,738 total individuals) who served on active duty in the military during the Vietnam conflict (1964-1975). The Registry also maintains a register, data repository, and a biospecimen repository. Details on the operations of the Registry are described, as well as an overview of specific studies. Registry maintenance activities are also described, including the updating of contact information and vital status. Future plans include expanding the biospecimen repository and obtaining input from twins about study methods and diseases and conditions they would like to see investigated.