Multipotent neural progenitor cells (NPCs) undergo self-renewal while producing neurons, astrocytes, and oligodendrocytes. These processes are controlled by multiple basic helix-loop-helix (bHLH) fate determination factors, which exhibit different functions by posttranslational modifications. Furthermore, depending on the expression dynamics, each bHLH factor seems to have two contradictory functions, promoting NPC proliferation and cell-cycle exit for differentiation. The oscillatory expression of multiple bHLH factors correlates with the multipotent and proliferative state, whereas sustained expression of a selected single bHLH factor regulates the fate determination. bHLH factors also regulate direct reprogramming of adult somatic cells into neurons and oligodendrocytes. Thus, bHLH factors play key roles in development and regeneration of the nervous system. Here, we review versatile functions of bHLH factors, focusing on telencephalic development.