Natural Gas engines are viewed as an alternative to diesel power in the quest to reduce heavy duty vehicle emissions in polluted urban areas. In particular, it is acknowledged that natural gas has the potential to reduce the inventory of particulate matter, and this has encouraged the use of natural gas engines in transit bus applications. Extensive data on natural gas and diesel bus emissions have been gathered using two Transportable Heavy Duty Vehicle Emissions Testing Laboratories, that employ chassis dynamometers to simulate bus inertia and road load. Most of the natural gas buses tested prior to 1997 were powered by Cummins L-10 engines, which were lean-burn and employed a mechanical mixer for fuel introduction. The Central Business District (CBD) cycle was used as the test schedule. The potential of these natural gas engines to reduce emissions significantly was demonstrated, but vehicles that had strayed from a proper state of tune were found to emit high levels of carbon monoxide, and could emit oxides of nitrogen at higher levels than their diesel counterparts, emphasizing the need for careful maintenance. Recently, one of the Transportable Laboratories tested ten compressed natural gas and three diesel buses in Atlanta, GA and seven diesel buses in Flint, MI. The natural gas vehicles were powered by Detroit Diesel Series 50 G 275hp engines, which employ lean burn, with electronically managed fueling without feedback control. CBD cycle emissions data showed that the natural gas buses offered lower levels of oxide of nitrogen emissions than their diesel counterparts, and that their particulate emissions were exceptionally low. Oxides of nitrogen emissions ranged from 26.1 to 60.4 grams/kilometer while carbon monoxide emissions were consistently close to 14.5 grams/kilometer for all of the Atlanta natural gas buses. Data are also available from these buses to show the methane versus non-methane split of hydrocarbons, and data also show the emissions reduction associated with correcting an ignition system fault on one bus. In addition, the effect of the test cycle on diesel bus emissions has been discussed.