Add Health is a nationally-representative, longitudinal survey of adolescents initiated in 1994. It began with an in-school questionnaire administered to students in grades 7-12, followed by in-home interviews in 1995, 1996, 2001-02, 2008, and 2016-18. The study has collected data on demographic characteristics, family education, physical and psychological health, social networks, substance use, reproductive health, and biomarkers. Data is available in public-use and restricted-use formats for Waves I-V, with the public-use dataset comprising half of the core sample and half of a specific oversample of African-American adolescents. Wave I included 6,504 respondents, Wave II re-interviewed 4,834, Wave III re-interviewed 4,882, Wave IV included 5,114 respondents, and Wave V included 4,196 respondents. Additional data includes contextual information on communities and states, parental survey data, and genetic/biological health markers.