A cooperative group of institutions, investigators, and other collaborators organized to evaluate treatment for HIV-infected children, adolescents, and pregnant women, including treatment and prevention of coinfections; to develop and evaluate safe and effective approaches to prevent mother-to-child transmission of HIV; and to evaluate vaccines for the prevention of HIV transmission among adolescents. International Maternal Pediatric Adolescent AIDS Clinical Trials (IMPAACT) Group was formed through a merger of investigators, including investigators from the Pediatric AIDS Clinical Trials Group (PACTG).