BYDS investigates substance use behaviours (particularly problem drug use) from early adolescence to adulthood. The first phase of BYDS included five annual waves of data collection of cohort members between 10 and 15 years of age. There were 4,411 pupils in the participating schools, 3,834 (87%) of whom completed a baseline questionnaire. The second phase of BYDS included two waves of data collection (waves 6 and 7) during which participants were aged 16 to 21 years, transitioning from compulsory schooling to further education and employment.