A study of over 59,000 Icelandic adolescents by a team of Icelandic and North American behavioral and social scientists found that COVID-19 has had a significant, detrimental impact on adolescent mental health, especially in girls. The study is the first to investigate and document age- and gender-specific changes in adolescent mental health problems and substance use during the COVID-19 pandemic, while accounting for upward trends that were appearing before the pandemic.