Biodiesel is a renewable transportation fuel consisting of fatty acid methyl esters (FAME), generally produced by transesterification of vegetable oils and animal fats. The effects of biodiesel usage upon vehicle emissions have been investigated by numerous groups. A consensus view has developed that emissions of hydrocarbons (HC), carbon monoxide (CO) and fine particulate matter (PM) can be reduced by use of biodiesel, while oxides of nitrogen (NOx) increase slightly.This paper provides a review of the literature regarding the effects of biodiesel upon emissions of these four criteria pollutants. The emissions database was restricted to studies in which both biodiesel and a conventional diesel fuel were tested under identical dynamometer conditions. Both heavy-duty (HD) and light-duty (LD) engines/vehicles were considered. The biodiesel emissions results were computed as percent difference from the base fuel results, which enables investigation of fuel effects across a wide variety of engine types, operating conditions, and biodiesel blend levels.In agreement with previous reviews, use of biodiesel was found to provide emissions reductions for CO, HC, and PM - though the effects vary somewhat with engine/vehicle type. NOx effects are small and variable. For B20 blends, efforts were made to identify effects of biodiesel type (soy, rapeseed, palm, etc.), base fuel, engine technology, and test cycle upon the criteria emissions. Due to high variability, effects of these parameters could not be clearly differentiated.