Microglia are the resident monocytic cells in the central nervous system (CNS), where they constitute the complex tissue structure together with a diverse set of cell-types, including neurons, glial cells, and vasculature. Different from other cells, microglia have distinct features, including not only their origin but functions in the CNS, and serve multiple roles within healthy and diseased CNS tissue. The present review highlights the latest advance in our understanding of the nature of microglia in the CNS, from their origin to both the physiological and pathological roles throughout their whole life.