The development and operation of efficient, low-polluting marine diesel engines is the major goal of researchers, manufacturers, and the International Maritime Organization. Mandatory controls on the exhaust emissions from marine sources have now radically and fundamentally changed with the adoption of Annex VI Regulations for the Prevention of Air Pollution from Ships to the Marine Pollution (IMO MARPOL 73/78) Convention. Nitrogen is practically inactive. Only a small but an important part is involved in the chemical reactions in the engine, forming NOx emissions. This study originally indicates the NOx control methods that are affecting the engine combustion process directly and reducing the emission level without changing the engine performance characteristics in marine diesel engines. The different applied methods and combinations of new fuel injection valve types, water injection, exhaust gas recirculation, and the use of selective catalytic reduction ensure the significant decrease in accordance with the international and national standards.