The cellular compartment of the central nervous system (CNS) consists of neurones and glia. Whilst neurones fire electrical impluses to transmit messages, glia were classically thought to be mere 'connective tissue' of the brain. This notion no longer holds true. Astrocytes, oligodendrocytes and microglia are now known to have specific functions within the complex CNS microenvironment. Microglia are the resident macrophages of the CNS. They scavenge brain tissue, acting as sensors that monitor the well being of the CNS milieu. When the brain is stressed or injured, microglia assume an activated phenotype to launch an inflammatory attack on the assailant. To do this, activated microglia proliferate, upregulate expression of inflammatory receptors, produce inflammatory mediators and become phagocytic.