This book explains the benefits and challenges associated with internal combustion, compression ignition, gas-fueled, and premixed dual-fuel engines. It explores methane and natural gas as engine fuels, as well as liquefied petroleum gases, hydrogen, and other alternative fuels. It examines safety considerations, combustion of fuel gases, and the conversion of diesel engines to dual-fuel operation. It addresses dual-fuel engine combustion, performance, knock, exhaust emissions, operational features, and management. It also describes dual-fuel engine operation on alternative fuels and the predictive modeling of dual-fuel engine performance.