The Finnish Twin Cohort comprises three different cohorts: the Older Twin Cohort established in the 1970s, as well as the FinnTwin16 and FT12, both initiated in the 1990s. FT12 is the youngest of the three Finnish Twin Cohorts. FT12 was designed as a two-stage study. In the first stage, a multi-wave questionnaire was conducted, enrolling all eligible twins born in Finland during 1983 to 1987, along with their biological parents. In stage 2, a subset of these twins was intensively studied with in-school assessments at age 12 and semi-structured poly-diagnostic interviews at age 14. At baseline, parents of intensively studied twins were also administered the adult version of the interview. The basic aim of the FT12 study design was to obtain information on individual, familial, and school/neighbourhood risks for substance use/abuse prior to the onset of regular tobacco and alcohol use and then track trajectories of use and abuse and their consequences into adulthood.