Microglia (MG) are enigmatic cells of the central nervous system (CNS). MG are morphologically, antigenically and functionally flexible, and have the potential for mobility and proliferation. MG are professional antigen-presenting cells and constitute part of the local CNS innate immune system, communicating with other immune cells via chemokines, cytokines and growth factors. MG contain several antigenic and functional markers similar to macrophages and dendritic cells (DCs), but also present several differences from DCs. The exact role(s) played by MG in the normal human CNS is the topic of lively debate. MG participate in many reactive processes in the CNS and are therefore an integral part of lesions in a variety of pathologic conditions. It is thought that MG may exacerbate diverse neurological conditions, including viral encephalitis, AIDS, Multiple Sclerosis (MS) and Alzheimer's disease. A recurrent theme is the perpetuation by MG of pathological cycles of monocyte recruitment, activation and cytopathic secretions, and/or auto antigen presentation.