 The twins' early development study, TEDS, is a long-term research project studying the developmental trajectories of twin pairs from birth through adolescence. The study has followed a cohort of twins born between 1994 and 1996 in England and Wales since their parents first agreed to participate. Over 10,000 families have remained enrolled in the study, providing valuable information about the genetic and environmental factors influencing mental health in young adults. Recently, the TEDS team has been collecting data on mental health in mid-20-somethings, including a comprehensive assessment of mental health at age 26 and a multi-phase study on the effects of COVID-19 on mental health. Additionally, they are working with the children of TEDS, Co-TDS, project to link data from the original TEDS participants with their children's data. This article was authored by Celestine Lockhart, Joanna Bright, Yasmin Ahmadzada, and others.