Glia are non-neuronal cells with diverse functions that range from forming the myelin sheath to defending the brain against infection. A major goal of our research is to use the powerful experimental advantages of zebrafish to discover new genes that are essential for the development and function of two classes of glia in the CNS, oligodendrocytes and microglia. Oligodendrocytes form myelin on axons in the CNS. After an oligodendrocyte begins to myelinate axons, it has only a short developmental window (or “critical period”) to extend new myelinating processes. Using genetic and cellular approaches in zebrafish, we have identified a number of positive and negative regulators of myelination. One of our goals is to determine how these factors control myelination during development, neural plasticity, and remyelination. In addition, we will investigate the molecular basis of the critical period. Microglia are highly motile, phagocytic glial cells in the CNS that destroy pathogens and clear debris such as apoptotic cells and damaged axons. Despite the importance of microglia in CNS health and disease, many critical questions remain to be addressed about these cells. We have conducted zebrafish mutational screens to discover essential microglial genes, and we are characterizing their functions using in vivo imaging and other approaches. The mechanistic insight gained from these studies will advance our fundamental understanding of the central nervous system, illuminate the pathways that are disrupted in diseases of the brain, and suggest avenues toward therapies for neurological disorders.