The Nicotine Dependence in Teens (NDIT) study is an ongoing 22-year longitudinal investigation of 1294 students recruited in 1999-2000 from all grade 7 classes in a purposive sample of 10 high schools in Montreal, Canada. Its primary objectives were to study the natural course and determinants of cigarette smoking and nicotine dependence in youth, although the project also embeds studies on obesity, blood pressure, physical activity, team sports, exergaming, sedentary behaviour, diet, genetics, alcohol use, cannabis use, use of e-cigarettes, use of illicit drugs, exposure to second-hand smoke, gambling, sleep and mental health. The main source of data was self-report questionnaires administered in class at school every 3 months from grade 7 to grade 11 (1999–2005), for a total of 20 cycles during high school. Questionnaires were also completed after graduation from high school in 2007–08, 2011–12, and 2017-20 (cycles 21, 22 and 23, respectively) when participants were age 20, 24 and 31 years on average, respectively. Cycle 24, which collected data on COVID-19 is was undertaken from December 2020 to June 2021, and Cycle 25 is ongoing. Data collection also comprised anthropometric measurements, DNA sampling, parental questionnaires and school environmental scans.