Microglia are the resident immune cells of the brain, and are located within the brain parenchyme behind the blood-brain barrier. They originate from mesodermal hemapoietic precursors and are slowly turned over and replenished by proliferation in the adult central nervous system. In the healthy brain resting, ramified microglia function as supportive glia cells, and their activation status is regulated by neurons through soluble mediators and cell-cell contact. However, in response to brain pathology microglia become activated: acquisition of innate immune cell functions render microglia competent to react towards brain injury through tissue repair or induction of immune responses. In certain pathological conditions, however, microglia activation may sustain a chronic inflammation of the brain, leading to neuronal dysfunction and cell death. This might be mediated by the microglial release of extracellular toxic reactive oxygen and nitrogen species. Nevertheless, in the future microglia may potentially be harnessed for therapeutical purposes.