Hypothesis 1.1 Controlling for potential confounding factors such as substance use and length of infection, there will be an interaction between effects of age and HIV serostatus on neuropsychological performance, and this will be evident both cross-sectionally and longitudinally. Specifically, we expect to find that older HIV+ individuals will exhibit greater rates of neuropsychological impairment (using age-corrected test norms) than younger HIV+ persons. Neurocognitive functions subserved by frontal-subcortical systems that are sensitive to the effects of both aging and HIV infection (learning, motor and psychomotor speed, executive function) will be disproportionately affected among the older HIV+ participants. While the synergistic effects of HIV and age will be evident on a cross sectional basis, they will be most pronounced when examined longitudinally over the course of the study.