The Colorado Twin Registry (CTR) is a population-based registry housed at the Institute for Behavioral Genetics at the University of Colorado. Recruitment began in 1982 and includes twins born from 1968 to the present. Four samples are currently drawn from the CTR: The Community Twin Sample, the Longitudinal Twin Sample, the Early Reading Development Sample, and the Colorado Learning Sample. Criteria for enrollment, recruitment strategies, demographic information, and zygosity assignment are explained for each sample. In addition, five studies in which CTR twins are now participating are highlighted. These include studies of cognition, learning ability, and vulnerability to substance abuse and antisocial behavior. The development of the CTR is an ongoing and evolving process, and it has proven to be a valuable registry, relatively representative of the population from which it was drawn.