Aims: This paper aims to give a description of the Danish National Youth Study 2014 in terms of study design, study population and questionnaire content. The differences between participants and non-participants regarding socioeconomic characteristics are also described. Methods: The Danish National Youth Study 2014 was a web-based survey with data collected through self-completion questionnaires administered in the classroom. There were two questionnaires: one for students, with >250 core questions; and one for school leaders on the school environment. Data collection took place at 119 high schools and 10 vocational schools. Results: A total of 75,853 students participated (70,674 high school students and 5179 vocational school students). In the participating schools, 85% of high school students and 69% of vocational school students took part in the survey. A total of 166 school leaders responded. Among the high school students, 61% were girls, and among vocational school students, 24% were girls. The average age was 17.9 years for high school students and 20.9 years for vocational school students. Participants were more likely than non-participants to be of Danish origin and to have parents with higher educational levels and a higher disposable income. Conclusions: The Danish National Youth Study 2014 contributes to knowledge on adolescent health behaviour, health and well-being. It is unique in its size, diversity of questionnaire content, high participation rate and possibility of linkage to various national registers through the Danish Civil Registration System. The study offers great opportunities for health planning and a wide range of future research projects.