Controlling emissions is becompping a more and more important task in the development of diesel engines. So far the most stringent emission limits are valid for landbased power plants. But today the International Maritime Organization (IMO)  is close to completing regulations prescribing the upper limits for nitrogen oxides (NOx) emissions from engines running on ships. The new limits are based on the rated engine speed.This paper mainly discusses the results of the research efforts made by New Sulzer Diesel Ltd to achieve the required NOx reductions. The solutions comprise both primary and secondary measures. The various primary measures aim at reducing NOx at the source - the combustion process - whereas secondary measures reduce the NOx emissions produced from the engine by exhaust aftertreatment.The results show that by the year 2000, it is expected NOx limits imposed by the IMO can be met by using well chosen primary methods that are more economical than secondary methods.