Abstract When plug-in electric vehicles (PEVs)—plug-in hybrids and battery electric vehicles—reach mass market adoption, their growth may be self-sustaining and their replacement of fossil-fuel vehicles nearly certain. Understanding this milestone is important, because afterward research and policy interventions that support PEVs will be less necessary. In this paper we outline how to identify mass market adoption and show that the PEV market has not reached this point based on two essential criteria: achieving adoption (not sales) of about 16% and the resolution of early market issues (any deficiencies affecting vehicles, infrastructure, and the surrounding ecosystem). The fact that neither of these criteria have been met highlights the early stage of the PEV market and means PEVs will require policy support throughout this decade.