Most plant and seed (vegetable) oils can be used directly as diesel engine fuels without any processing, but their physical properties can cause problems with fuel injection and emission control systems, and leave cylinder deposits. Biodiesel fuel is usually blended with mineral diesel fuel to a maximum percentage of 20% (B20). Renewable diesel fuel is a direct replacement for mineral diesel fuel that is 100% compatible but is more expensive than biodiesel. Biodiesel and renewable diesel fuels were developed to replace mineral (petroleum distillate) diesel fuel for automobiles and trucks. They have properties sufficiently similar to mineral diesel to allow partial or complete substitution, respectively. Both qualify for incentives under U.S. Renewable Fuel Standard (RFS) Program and may also qualify for incentives in some states. This chapter examines the production and use of each of these biofuels for compression ignition engines, including their operational characteristics, full-cycle energy economics, and environmental impacts.