The twin study project at the University of Helsinki began in 1974 with the Older Finnish Twin Cohort study in order to investigate genetic and environmental determinants of common, complex diseases and their behavioural risk factors Later two younger birth cohorts, Finntwin16 (twins born 1975-79) and Finntwin12 (born 1983-1987) studies were initiated. These longitudinal follow-up studies have continued to this day. Within these cohorts, smaller studies with more detailed data collection (clinical, imaging, -omics, for example )have been conducted. Currently, the Twin Study project is led by Professor Jaakko Kaprio, and the research group is working together with many researchers in Finland and internationally.