The operation of engines whether spark ignition or compression ignition on a wide range of alternative gaseous fuels when using lean mixtures can offer in principle distinct advantages. These include better economy, reduced emissions, and improved engine operational life. However there are distinct operational mixture limits below which acceptable steady engine performance cannot be sustained. These mixture limits are usually described as the "lean operational limits, " or loosely as the ignition limits which are a function of various operational and design parameters for the engine and fuel used. Relatively simple approximate procedures are described for predicting the operational mixture limits for both spark ignition and dual fuel compression ignition engines when using a range of common gaseous fuels such as natural gas/methane, propane, hydrogen, and some of their mixtures. It is shown that good agreement between predicted and corresponding experimental values can be obtained for a range of operating conditions for both types of engines.