Environmental and economic factors are contributing to a growing market for vehicular engines fueled by natural gas. Engine users are moving from general curiosity in alternative fuels and their low emissions potential to demanding Original Equipment Manufacturers to provide such engines that meet their operational requirements.Customer needs of performance, reliability and durability from natural gas engines are requiring the engine manufacturers to develop appropriate technologies to achieve these objectives. This paper describes some of the technologies developed at Cummins Engine Company for the B5.9G natural gas engine. The lean burn concept developed for the engine and its advantages are described in addition to the techniques utilized to implement the technology for vehicular applications. Combustion, air handling and fuel metering systems design and development are outlined as well as considerations to minimize engine performance sensitivity to the gas supply system selection (CNG, LNG) and gas composition are discussed.The design and performance of the ignition system is also presented. Effects of in-cylinder conditions and design parameters on ignition system performance and durability are also included.The engine electronic control system is discussed and its elements described including the open-loop and closed loop features of the air/fuel ratio control strategy and its advantages. In addition, performance and emissions data are presented to highlight the overall system behavior.