Current and proposed emissions standards in the United States, Europe and Japan are creating unique markets for the introduction of new powertrain technology. Adding to the complexity of the evolving tailpipe emission standards are differing vehicle and dynamometer test cycles and increased emphasis on CO2 reduction and higher vehicle fuel economy. In addressing the challenges posed by increasingly more stringent emissions standards and demands for high efficiency powertrain technologies, partitioning the tailpipe emissions requirements has the potential to identify and dimension significant design, development and systems tasks.This paper describes the use of emissions index, the ratio of emissions mass flow to fuel mass flow, to define the tailpipe emissions capabilities required from engine/fuel/calibration and after-treatment/control systems. When the tailpipe standard and vehicle fuel economy are fixed, engine out emissions index and after-treatment/control system effectiveness are uniquely related. The resulting partitioning is not perfect. Some base engine design and fuel characteristics, which do not strongly influence engine out emissions, can affect the performance of control and after-treatment systems. However in spite of this fuzziness, use of emissions index provides a high level structure for analyzing vehicle systems and setting development priorities.Finally, the paper examines the development challenges of selected North American, European and Japanese passenger car tailpipe emissions standards. Comparisons are made that illustrate the magnitude of changes required to move a technology developed for one market or emissions standard to others. The implications of LEV2 for direct injection compression ignition engines and direct injection stratified charge, spark ignition engines with lean NOx after-treatment systems are discussed.