Biodiesel has been defined as the monoalkyl esters of long-chain fatty acids derived from renewable feedstocks, such as vegetable oils or animal fats, for use in compression-ignition (diesel) engines. Biodiesel has become more attractive because of its environmental benefits and the fact that it is made from renewable resources. Biodiesel is the first and only alternative fuel to petroleum diesel to have a complete evaluation of emission results. The emission-forming gasses, such as carbon dioxide and carbon monoxide from combustion of biodiesel hydrocarbons, generally are less than diesel fuel. Sulfur emissions are essentially eliminated with pure biodiesel. The exhaust emissions of sulfur oxides and sulfates from biodiesel were essentially eliminated compared to diesel. The smog-forming potential of biodiesel hydrocarbons is less than diesel fuel. The ozone-forming potential of the speciated hydrocarbon emissions was 50% less than that measured for diesel fuel.