An overview of recent studies of the impact of biodiesel and biodiesel blends on air pollutant emissions and health effects is provided. Biodiesel blends of 20% produce reductions of 15% or higher (depending upon engine model and test cycle) in emissions of particulate matter, carbon monoxide, total hydrocarbons, and a group of toxic compounds including vapor-phase hydrocarbons from C1 to C12, aldehydes and ketones up to C8, and selected semivolatile and particle-phase PAH and NPAH. Based on the studies reviewed and recently acquired data, individual engines may show oxides of nitrogen increasing or decreasing, but on average there appears to be no net effect for blends of 20% biodiesel--the most common biodiesel blend. Exhaust from a diesel engine operating on 100% biodiesel was also shown to have only modest adverse effects in an animal exposure study. Studies of the impact of biodiesel on particle size have not produced consistent results and additional research in this area is needed. Biodiesel is also shown to significantly reduce life-cycle greenhouse gas emissions in comparison to petroleum diesel.