With the aim of improving the air quality in large cities, the California Air Resources Board (CARB) has stipulated that non-methane organic gas (NMOG) composed of carbon numbers from C1 to C12 must be reduced for vehicle categories designated as Transitional Low Emission Vehicles (TLEVs), Low Emission Vehicles (LEVs), Ultra low Emission Vehicles (ULEVs), and Zero Emission Vehicles (ZEVs). Although considerable research work has been done on this issue to date, the entire picture is still not clear. Studies done by the authors have been aimed at providing a better understanding of the potential for reducing automotive tailpipe emissions by using several clean fuel candidates.The major questions of concern are the extent to which emissions of certain species can actually be reduced and what fuel can provide the best performance under a reduced NMOG condition. The technological assessment includes different ways of investigating mass emissions, groupings of hydrocarbons each having a general formula and particular species cited as being toxic air pollutants. In addition, an examination is also made of the possibility of reducing automotive carbon dioxide emissions simultaneously, which is required from the standpoint of global environmental protection.The possibility of meeting emission requirements through near-future developments is beyond the scope of this discussion.