Microglia are the resident brain immune cells of the central nervous system (CNS). These glial cells have an essential role in the CNS development and homeostasis, as well as in several diseases through the production of proinflammatory cytokines, trophic factors and reactive oxygen species, among other elements. Microglia cells are one of the most intrinsically plastic cell types of the human body, possessing the ability to assume different morphologies and molecular properties in response to their rapid-changing surroundings. This book discusses the physiology, regulation and health implications of microglia.