Microglia can become motile and actively move to a lesion or herd of infectious invaders following chemotactic gradients. Local densities of microglia can also increase by proliferation, to provide more cells for the defense against invading germs and to organize for the protection and restoration of tissue homeostasis. Induction and rearrangement of surface molecules for cell-cell and cell-matrix interactions, changes in intracellular enzymes as well as release of multiple factors and compounds with proinflammatory and immunoregulatory effects are additional elements of the activation process. Microglia can unfold their phagocytotic activities to clear tissue debris, damaged cells, or microbes. Release of chemoattractive factors recruits and guides immune cell populations to the CNS, and presentation of antigens to T cells can subsequently aid the adaptive immunity in fight against viral or bacterial invasion. The range of microglial activities also covers production of neurotrophic factors and the physical association with endangered neurons.