The traditional role of microglia has been in brain infection and disease, phagocytosing debris and secreting factors to modify disease progression. Recent evidence extends their role to healthy brain homeostasis, including the regulation of cell death, synapse elimination, neurogenesis, and neuronal surveillance. These actions contribute to the maturation and plasticity of neural circuits that ultimately shape behavior. Here we review microglial contributions to the development, plasticity, and maintenance of neural circuits with a focus on interactions with synapses. We introduce this topic by reviewing recent studies on the migration and proliferation of microglia within the brain, and conclude with the proposal that microglia dysfunction may adversely affect brain function, and thereby contribute to the development of psychiatric and neurological disorders.