This paper analyzes data obtained from international studies that compared natural gas, diesel, and hybrid technologies for buses to determine differences in particulate matter (PM10), nitrogen oxides (NOx), nonmethane hydrocarbon (NMHC), carbon monoxide (CO), greenhouse gas, and air toxic emissions. The results presented here are for 90-passenger 12-m buses. Two types of comparisons were made: technology and fuel. The comparison revealed that buses using ultra-low-sulfur diesel (ULSD) with diesel particulate filters (DPF) and buses using compressed natural gas (CNG) with oxidation catalysts (OC) had the lowest emissions of PM10 and CO; CNG with OC had lower NOx and greenhouse gases emissions, while ULSD with DPF had lower NMHC emissions. Posttreatment technologies are presented as a low-cost option to reduce emissions. Hybrid diesel buses were found to be the technology that has the lowest emissions for most of the studied pollutants.