During brain development, neural stem cells gradually change their competency, giving rise to various types of neurons first and glial cells later. It is thus very important to maintain neural stem cells until the final stage of development to generate a full diversity of cell types. We found that expression of the basic helix-loop-helix (bHLH) factor Hes1 oscillates in neural stem cells, and that Hes1 oscillation drives the cyclic expression of proneural factors such as Ascl1/Mash1. During neuronal differentiation, Hes1 expression disappears and proneural factor expression becomes sustained. By contrast, during astrocyte differentiation, Hes1 expression becomes dominant while proneural factor expression disappears. These results suggest that the multipotency is a state controlled by multiple oscillating fate-determination factors, and that one of them becomes dominant during fate choice. We further showed by optogenetic approach that sustained expression of Ascl1 promotes neuronal differentiation, whereas oscillatory expression of Ascl1 activates proliferation of neural stem cells, indicating that the expression dynamics is important for the function of Ascl1. We now examine the dynamics of Hes1 and Ascl1 in the adult brain and will discuss how these factors regulate adult neurogenesis.