Abstract According to some definitions, an energy transition refers to the time that elapses between the introduction of a new primary energy source, or prime mover, and its rise to claiming a substantial share of the overall energy market. According to one academic view, energy transitions take an incredibly long time to occur. Another view argues the opposite. It suggests that there have been many transitions at varying scales that have occurred quite quickly—that is, between a few years and a decade or so, or within a single generation. This chapter holds that both sides are partly right, and partly wrong. After presenting evidence in support of either thesis, it elucidates four lessons for energy analysts and policymakers.