The US 1991 and 1994 emission standards for heavy duty diesel engines present the industry with one of the most difficult challenges to date. Achieving the 1994 standards in particular will require extensive development using an integrated approach. All aspects of the engine will need to be addressed including the boost strategy, the combustion system, oil control and exhaust aftertreatment. Engines will need to be fully optimised for transient operation. In this paper, the 1994 technology requirements for highly rated, heavy duty diesel engine's are reviewed.