Urban transit bus fleets are a significant source of air pollutant emissions, including black carbon, a harmful ultrafine particle and potent short-lived climate pollutant. Transit bus fleets are therefore an important target for accelerated transitions to clean engine technologies and fuels. A number of engine technologies and enabling fuels are available today that achieve “soot-free” emissions performance—the near elimination of emissions of black carbon and other harmful pollutants. However, there are open questions as to the relative climate emissions impact of the various technologies and how it varies across cities and within cities depending on the operating mode of the bus. To address these questions, this working paper evaluates well-to-wheel (WTW) greenhouse gas (GHG) emissions from commercially available soot-free transit buses in 20 megacities. Bus types include diesel, diesel-electric hybrid, and compressed natural gas (CNG) buses powered by engines certified to Euro VI or EPA 2010 emission standards, as well as battery electric buses. For diesel and CNG buses, GHG emissions are calculated for conventional fossil fuels, as well as select biofuel alternatives.