On average, each year over the last decade about 900 diesel engines with total power capabilities approaching 7,000 MW have been produced for civilian shipping applications. Diesel engines are also used extensively in naval applications especially in submarines where the diesel-electric vehicles still dominate the market some forty years after the advent of nuclear propulsion. Until recently, environmental air quality concerns were not a major issue in the design of such marine engine systems. Consequently, although a great deal of contemporary diesel engine research and development for land based applications, particularly transportation, has focused on measures to reduce “health risk” exhaust emissions, other factors have tended to dominate marine engine design developments.The ability to handle variable fuel quality in a cost efficient manner has been a main theme in marine diesel engine research especially for commercial markets. On the military side the need for high power to weight units which are ultra-reliable and as quiet as possible has been a paramount requirement. However, a growing body of legislation is now starting to include marine engine emissions in regulated environmental protection. Of the major identified pollutants in diesel engine exhausts, i.e., carbon monoxide, smoke, hydrocarbons, particulate matter (PM) and NOx, the control of the latter two has created the most significant challenges.European and Japanese diesel engine manufacturers dominate the world market and they, over the last few years, have started to investigate ways of not only satisfying current air quality regulations but of reducing concentrations below required levels. The object of this research thrust is to ensure that an engine can be operated over an extended lifetime with reduced maintenance and yet meet any future emission laws. To a large extent this is the same direction that US diesel manufacturers have chosen for their land based units. Thus, exhaust aftertreatment, and intake charge modifications by cooling or recycling have all been investigated.In this paper, the international efforts in these major areas of diesel engine research are reviewed with specific regard to NOx emissions. Some of the current research being undertaken by the authors on NOx emissions from indirect injection engines is also discussed.