The effects of hydrogen addition on the ignition and combustion of a high-pressure methane jet in a quiescent charge of high-temperature, medium-pressure air were investigated numerically and experimentally. Subsequently, the results of these two fundamental studies were applied to the interpretation of combustion and emissions measurements from a pilot-ignited natural gas engine fueled with similar fuels. Whereas, under quiescent conditions, the influence of hydrogen addition on the autoignition delay time of the gaseous jet was small, a markedly greater effect was observed in the more complex environment of the research engine. Similarly, in the two fundamental studies, the addition of hydrogen to the methane fuel resulted in a reduction of NOx emissions, whereas increased levels of NOx emissions were observed from the engine, highlighting the difference between the autoignition and pilot-ignition process.