City buses can utilise the existing road infrastructure much better than passenger cars and any other type of vehicle for passengers transportation on road. The emissions of greenhouse gases, such as carbon dioxide, from buses are significantly lower than from passenger cars. However, since the exhaust emissions from petrol-fuelled cars has decreased so dramatically during the last decade, the emissions of several emission components, e.g. NOX and particulates, from city buses are higher than from the cars. Alternative fuels could play a role in reducing the exhaust emissions in comparison to conventional diesel buses, thereby improving the local air quality. Several different alternative fuels, such as e.g. natural gas, biogas and ethanol, have been utilised during the last decade. New fuels as dimethyl ether (DME) and hydrogen are in discussion for the future but these fuels are greatly dependent on the future development in areas such as fuel production, fuel distribution and energy converters. Recently, the Swedish truck and bus manufacturer Scania declared that they would end the production of ethanol-fuelled buses leaving natural gas and biogas as the two main short-term options. Natural gas and biogas have been considered as inherently “clean” fuel options with considerable potential for further development. Low levels of NOX and particulate emissions are two main advantages. However, the introduction of cleaner diesel fuel and the use of aftertreatment devices such as, e.g. catalytic particulate filters have also decreased the emissions from these buses. Recently, the emissions from buses fuelled by natural gas have been in the focus in the USA. There are relatively few recent data on emissions from gaseous- fuelled city buses in Sweden. The scope of this work has been to summarise the experiences from international activities and, based on the knowledge gained, propose a test programme for tests on gaseous-fuelled buses and their diesel-fuelled counterparts.