With the development of large twin-family registers worldwide (see the December 2006 issue of Twin Research and Human Genetics for an overview) new options for research projects become feasible, including longitudinal studies. In this special issue on Longitudinal Twin and Family Studies, several aspects of longitudinal research are highlighted and an overview of results is given from European, Australian and United States twin studies. These studies deal with a time span from birth to adulthood and a wide range of phenotypes is covered. Data collection in some studies is age/cohort driven; in others, subjects are followed from different ages at regular time intervals. Most studies are based on data collection in twins, but adoption and parent–offspring data designs are described as well. The overview of results of current longitudinal projects makes this special issue a reference resource for longitudinal research in developmental behavior genetics.