Microglial cells, the resident macrophage population of the central nervous system (CNS), actively scan tissue under both normal and pathologic contexts. Their resulting engagement can become either neuroprotective or neurotoxic, leading to amelioration or aggravation of disease progression. In this review, we focus on the molecular signaling molecules involved in microglial responses and discuss observations demonstrating the diverse effects of microglia in animal models of CNS diseases.