IMAGEN combines brain imaging, genetics, and psychiatry with the aim to increase our understanding of adolescent brain development and behaviour – namely, sensitivity to rewards, impulsivity, and emotional processing. 14-year-old participants were recruited from high schools in the four study sites in England, France, Germany, and Ireland. To increase ethnic homogeneity in the sample, areas with minimal ethnic diversity were selected as recruitment sites. Eligible participants were recruited from private, state-funded and specialist schools to maximise socioeconomic, emotional and cognitive diversity within the sample. At baseline, the cohort included 2,000 adolescent participants. Participants and their parents have been followed from the age of 14, with follow-up assessments at ages 16, 19 and 22.