From the nineties there was a great interest in the use of compressed natural gas - CNG (predominantly composed of methane) on transit bus fleets around the globe. In a first moment, developed countries (US, EU and Japan) have focused their efforts to address serious urban air pollution problems caused by heavy duty diesel engines - since PM and NOx emissions were initially easier to control from natural gas engines than from conventional diesel engines - and also to offset growing oil imports. As such, for many years, dedicated methane fuelled city buses meeting emission requirements (Euro IV, V and EEV, US Federal and California, and Japan) either in a lean burn or stoichiometric technology, have been offered to the market. At the same time, stringent emission standards developed for new engines has stimulated industry in developed world to develop new diesel engine, aftertreatment and fuel (with low sulphur content) technologies, which has made modern electronic diesel competitive with CNG engines from an emission perspective. This, in some way, has waned the initial impetus for widespread adoption of CNG in transit markets. CNG transit strategy was also adopted by some developing countries, like India, due the impossibility to reach emissions limits already established (Euro III) with engines operated with diesel fuel with high sulphur content. Currently, with the advent of even stricter emissions targets - notably US 2010 and Euro VI limits - and the technical barriers to comply with their extremely low NOx limits with a diesel engine/aftertreatment approach - natural & biogas engines have been seen as a promising tool to reduce toxic emissions and mitigate CO2 emissions, specially when methane origins from biomass (sewage or landfill gas). Moreover, the current high availability and low cost of natural gas in US and some European countries as well as the perspective of near future increase of natural gas offer in developing countries, like Brazil, have fostered the interest of government and industry into methane powered heavy duty engines for transit service. This work is supposed to present an overview of technological approaches for Natural & Biogas (methane) heavy duty engines, their operational features and environmental performance, as well as a snapshot of current natural and biogas transit bus programs explored in some world cities.