Data has been gathered using the West Virginia University Heavy Duty Transportable Emissions Laboratories from buses operating on diesel and a variety of alternate fuels in the field. Typically, the transportable chassis dynamo meter is set up at a local transit agency and the selected buses are tested using the fuel in the vehicle at the time of the test. The dynamometer may be set up to operate indoors or outdoors depending on the space available at the site. Samples of the fuels being used at the site are collected and sent to the laboratory for analysis and this information is then sent together with emissions data to the Alternate Fuels Data Center at the National Renewable Energy Laboratory. Emissions data are acquired from buses using the Central Business District cycle reported in SAE Standard J1376; this cycle has 14 ramps with 20 mph (32.2 km/h) peaks, separated by idle periods. During the three years of testing, a significant fraction of emissions data was acquired from buses with Cummins L-10 engines designed to operate on either CNG or diesel. The CNG lean burn engines were spark ignited and throttled. Early CNG engines, which were pre-certification demonstration models, have provided the bulk of the data, but data from nine buses with more advanced technology were also available. It has been found that carbon monoxide (CO) levels from early Cummins L-10 CNG powered buses varied greatly from bus to bus, with the higher values ascribed to either faulty catalytic converters or a rich idle situation, while the later model CNG L-10 engines offered CO levels considerably lower than those typical of diesel engines. The NOx emissions were on par with those from diesel L-10 buses. Those natural gas buses with engines adjusted correctly for air-fuel ratio, returned very low emissions data. CNG bus hydrocarbon emissions are not readily compared with diesel engine levels since only the non-methane organic gases (NMOG) are of interest. Data show that NMOG levels are low for the CNG buses. Significant reduction was observed in the particulate matter emitted by the CNG powered buses compared to the diesel buses, in most cases the quantity captured was vanishingly small. Major conclusions are that engine maintenance is crucial if emissions are to remain at design levels and that the later generation CNG engines show marked improvement over the earlier models. One may project for the long term that closed loop stoichiometry control is desirable even in lean burn applications.