Microglia are resident macrophages of the central nervous system (CNS), representing 5-10% of total CNS cells. Recent findings reveal that microglia enter the embryonic brain, take up residence before the differentiation of other CNS cell types, and become critical regulators of CNS development. Here, we discuss exciting new work implicating microglia in a range of developmental processes, including regulation of cell number and spatial patterning of CNS cells, myelination, and formation and refinement of neural circuits. Furthermore, we review studies suggesting that these cellular functions result in the modulation of behavior, which has important implications for a variety of neurological disorders.