The major neurological complication of human immunodeficiency virus type 1 (HIV-1) infection is cognitive impairment, which can range in severity from a mild subclinical cognitive inefficiency to a severe dementing illness. Mild to moderate cognitive impairment is identified primarily by neuropsychological tests. The prevalence and severity of cognitive impairment associated with HIV-1 infection increases as the disease progresses. Deficits in attention, information processing speed, memory, and motor abilities can occur early in the course of HIV-1 infection, with deficits in abstraction and executive functions observed in later stages of infection. The nature of the cognitive impairment observed is thought to reflect the effects of HIV-1 infection on the integrity of subcortical or frontostriatal brain systems. Issues related to the detection of subclinical to severe cognitive impairment are discussed, along with the clinical significance of mild cognitive impairment as a significant risk factor for mortality in HIV-1 infection. The need to control for possible confounding factors that can influence test performance is also reviewed.