This collaborative research will generate new information on the development of treatment failure and drug resistance among a large, well-characterized cohort of HIV-infected children in western Kenya who receive common medications, using detailed, longitudinal, clinical, adherence, and drug concentration data. The proposed research is relevant to public health since such information is limited in the scientific literature, and withot it we cannot accurately assess treatment failure and drug resistance in HIV-infected children, which can lead to sickness and death. The long-term goal of this research is to provide evidence to improve clinical guidelines and chronic disease management for HIV-infected children in resource-limited settings.