The current article describes the creation and composition of the PennTwins Cohort and provides details on the demographic characteristics of the sample. The PennTwins Cohort was developed using a population-based method of ascertainment and currently has 9401 28- to 47-year-old twins, including 2225 confirmed twin pairs and 4951 twins whose co-twins have not yet registered. Zygosity data have been used to identify 919 monozygotic, 634 same-sex dizygotic, and 445 opposite-sex dizygotic pairs. GeoCode data on gender, age, and certain demographic characteristics were obtained for the addresses of all twins who were mailed invitations to be part of the cohort. Analysis of the available data show only very small differences between twins who are currently part of the PennTwins Cohort and potential twins who either did not respond to recruitment or who could not be located. Similarly, only very small demographic differences exist between twins from complete pairs and twins whose co-twins are not yet registered, and there are no differences across zygosity. Thus, despite a relatively low overall response rate (12% of all twins born in Pennsylvania from 1959–1978), there is no evidence that the sample differs in any meaningful respect from the larger population.