Add Health investigates the causes of adolescent health and health behaviour. The study recruited a core representative sample of young people aged between 12 and 19 in grades 7 to 12 from across the United States of America in the 1994 to 1995 school year. Rare and vulnerable populations, including ethnic minorities, adopted children, differently-abled children, monozygotic twins, dizygotic twins, and half siblings were over sampled to form a special sample with sufficient power for research. At baseline, the core sample consisted of 12,105 participants, and the special sample consisted of 8,640 participants, resulting in a total cohort of over 20,700 participants. Participants have been surveyed five times in 1994-95, 1996, 2001-02, 2008, and 2016-18. There are several substudies including the Add Health Parent Study and the Sexual Orientation/Gender Identity, Socioeconomic Status, and Health across the Life Course (SOGI-SES).