The formation of NOx in diesel engines is heavily influenced by the amount of available oxygen and the combustion temperature. This thesis investigates the effects of the implementation of exhaust gas recirculation on the NOx emissions of a 4-stroke marine diesel engine. With EGR implemented a part of the fresh intake air is replaced by exhaust gas. The exhaust gas contains less oxygen and has a slightly higher specific heat. This should result in a lower oxygen concentration and a lower combustion temperature. A mean value simulation model is used to simulate the engine and predictions of the NOx emissions will be made by using emission fits and certain engine parameters.