This is a milestone publication. Longitudinal datasets serve as a vital resource for researchers, providing invaluable insights into the complex interplay between the brain, body and environment in the trajectory and resolution of mental health problems. When Wellcome commissioned King’s College London, MQ Mental Health Research and the Open Data Institute to identify and gather information on promising longitudinal datasets from around the world we did not know exactly what they would find. The results of this important endeavour reveal the breadth of existing data that can be used for mental health research- the report identifies over 3,000 longitudinal datasets spanning 146 different countries.