HIV induces severe dementia in about 20% of adult AIDS patients. In children HIV-infected at birth, the incidence of specific neurological complications is still higher since severe encephalopathy occurs in almost all children who develop an early and severe immunosuppression. In all cases, the brain monocytes/macrophages and the microglial cells are the only cells which replicate HIV in the central nervous system (CNS) of these patients, and the appearance of neurological symptoms seems induced by an interaction between HIV-infected macrophages with neurons and glial cells. AIDS encephalopathy is related to two properties of HIV: to the viral tropism for monocytes/macrophages/microglial cells, which allow the brain infection, and to HIV tropism for CD4+ lymphocytes responsible for the appearance of immunosuppression, which trigger viral dissemination in the CNS. However, childhood encephalopathy is not always associated with HIV replication in the CNS at the time of death, and mild dementia in HIV-infected adults were described without signs of HIV replication in autopsy CNS samples. Those findings suggest that persistent, productive viral infection is not required for the development of HIV encephalopathy. Therefore, if the relationship between HIV CNS infection and AIDS encephalopathy in adults and children is clearly demonstrated, the pathogenesis of the neurological disease and the kinetics of HIV replication in the CNS are unclear. In addition, the very high incidence of AIDS encephalopathy in children could be related to HIV infection of microglia which is differentiating in fetal or newborn brain.