The nervous system evolved two basic mechanisms that enable great increases in impulse propagation velocity along axons. One is through axon gigantism and the second is through wrapping axons in multilamellar sheets, myelin sheath. My work aims to understand the developmental mechanisms that underlie myelin development in the vertebrate central nervous system. In the vertebrate central nervous system oligodendrocytes are responsible for forming myelin sheath. The developmental origin of oligodendrocyte progenitor cells (OPCs) and the composition of the myelin are well characterized, yet the mechanisms that underlie oligodendrocyte development in vivo are poorly understood. To investigate oligodendrocyte development, we took two approaches. One is to establish in vivo imaging system of oligodendrocytes using zebrafsih and another is to analyze the function of the genes expressed in oligodendrocytes by reverse genetic approach. In vivo imaging revealed the highly dynamic behavior of OPCs. On the other hand, reverse genetic approach by differential expression screen resulted in finding eight new genes that might be important in oligodendroyte development. In fact, the knockdown experiments for one of those, swap-70, indicate that it is involved in the regulation of OPC number and differentiation. Combining in vivo imaging and genetic analysis using zebrafish is a powerful approach for the study of oligodendrocyte development and myelin formation in vertebrates.