One of the most poorly understood aspects of animal development is how the timing of developmental events is controlled. In most vertebrate cell lineages, for example, precursor cells divide a limited number of times before they stop and terminally differentiate, but it is not known what controls when the cells stop dividing and differentiate. There is increasing evidence, however, that intracellular timers play an important part. Such cell-intrinsic timers are examples of intracellular developmental programs that change precursor cells over time. My colleagues and I have studied such intracellular timers and programs in rodent oligodendrocyte precursor cells (OPCs), as reviewed here.