Microglia are a major ghal component of the central nervous system (CNS) and are extremely sessile. Only a subtype, the perivascular microglia, are regularly replaced from the bone marrow in adult animals. Microglia respond to virtually any, even minor pathological events in the CNS. In most pathological settings microglia are aided by infiltrating hematogenous macrophages. Upon activation microglia and macrophages share most phenotypical markers and can exert similar effector functions. After transection of a CNS fibre tract microglia are insufficiently activated and hematogenous macrophages do not significantly enter the degenerating nerve stump. Thereby myelin debris that contains neurite outgrowth inhibiting activity persists for long time. This is in sharp contrast to the peripheral nervous system in which hematogenous macrophages are rapidly recruited in response to axotomy and clear myelin debris allowing regrowth of axons from the proximal stump. However, CNS lesion paradigms with breakdown of the blood-brain barrier such as cerebral ischemia, brain abscesses and stab wounds elicit prompt microglial activation, macrophage recruitment and debris clearance. There is increasing evidence that microglia play an active part in degenerative CNS diseases. In Alzheimer's disease activated microglia appear to be involved in plaque formation. In experimental globoid cell dystrophy T-cell independent induction of major histocompatibility complex class II molecules on microglia accelerates demyelination. In autoimmune diseases microglia probably have dual functions. Microglia present antigen to infiltrating T cells and exert effector functions thereby locally augmenting immune responses. On the other hand, microglia have the capacity to downregulate T cell responses. In the human acquired immunodeficiency syndrome (AIDS) virus infected macrophages probably introduce the virus to the CNS and in concert with microglia are involved in the pathophysiology of the AIDS dementia complex.