Over the years, natural gas has been promoted as a clean-burning fuel, especially for transit buses. A decade ago one could claim that natural gas buses deliver significant emission benefits over diesel buses, especially regarding particulate emissions. The spread in nitrogen oxide emissions has always been significant for natural gas engines, high for lean-burn engines and low for three-way catalyst equipped stoichiometric engines.With the introduction of US 2010 and Euro VI (effective as of 2014) exhaust emission regulations, independent of the fuel, the regulated emissions of all engines have been brought close to zero level. This means that the advantage of natural gas as a clean fuel is diminishing, especially in a situation in which electric transit buses are also entering the market.The motivation to use natural gas could still be diesel fuel substitution and to some extent, also reduction of greenhouse gas emissions. However, looking at the full fuel cycle and also taking into account methane leakages in various part of the fuel cycle, natural gas is, at its best, comparable to diesel regarding greenhouse gas emissions, not better. One of the reasons for this is the low fuel efficiency of spark-ignited gas engines compared to diesel engines. To deliver an advantage regarding greenhouse gas emissions, the efficiency of the gas engines should be significantly improved. Alternatively natural gas should be replaced with biogas.The paper presents a comparison of diesel and natural gas buses for regulated emissions, selected unregulated emission components, greenhouse gas emissions and energy efficiency.