In the twenty-first century, exhaust emission control will remain a major technical challenge especially as additional pressures for fuel and energy conservation mount. To address these needs, a wide variety of engine and powertrain options must be considered. For many reasons, the piston engine will remain the predominant engine choice in the twenty-first century, especially for conventional and/or parallel hybrid drive trains. Emissions constraints favor the conventional port fuel-injected gasoline engine with 3-way exhaust catalyst, while energy conservation favors direct-injection gasoline and diesel engines. As a result of recent technological progress from a competitive European market, diesels, and most recently, direct-injection (DI) diesels now offer driveability and performance characteristics competitive with those of gasoline engines. In addition, DI diesels offer the highest fuel efficiency. However, DI diesels will be strongly challenged by the emissions standards expected in the 2004-2005 period (e.g., Tier 2/ULEV and EC Stage IV). The technologies under development to enable compliance with these standards include optimized combustion chambers, high-pressure fuel-injection with electronic rate-shape control, and highly optimized systems to maintain high levels of EGR dilution. Compliance with future exhaust emissions standards, however, will also require improvements in exhaust aftertreatment. Fuel reformulation and additives may also play a role in meeting exhaust emissions challenges.