Development of more fuel efficient passenger vehicle engines based on lean burn technologies presents several challenges for controlling the emissions of regulated pollutants. Thus far, the emphasis has been on the selective reduction of NOx in the presence of excess O2. However, certification of vehicles based on lean burn technologies, whether gasoline or diesel, will require compliance with all regulated gas emission levels. Current working hypotheses on the origin of activity in lean NOx reduction catalysts will place strains on other aspects of the overall emissions system. Using the FTP-75 certification cycle as a basis for evaluation, the implications of using effective lean burn catalysts will be discussed. In particular, the requirement of ultralow hydrocarbon emissions has invoked strategies seemingly contradictory to the effective use of lean NOx catalysts. Possible alternative solutions and their consequences will be discussed.