The current low rates of HIV diagnosis and treatment initiation among adolescents and young people ages 15–24 continues to present a significant challenge to the epidemic control of HIV. With a ‘business as usual’ approach to HIV testing and linkage to treatment, new infections among adolescents and youth will likely increase, with the burden compounded by the increasing number of youth in Africa, expected to reach 293 million by 2025. Recent studies reveal significant gaps in the HIV clinical cascade among young people as the global community pursues the Joint United Nations Programme on HIV and AIDS 90–90–90 targets. This AIDS supplement was commissioned with the goal of informing program planners, researchers, policymakers, and funding agencies about the development and design of effective adolescent and youth programs, policies, and strategies for improving the first two 90s among adolescents and youth: HIV testing and diagnosis and linkage to care and treatment. Emerging evidence should inform efforts to better target the youth and adolescents who are most at risk, aiming for early diagnosis and treatment initiation for those who are HIV positive, while also ensuring appropriate primary prevention so that those identified as HIV negative remain so.