Concern about rising prices and unstable sources of petroleum fuels is driving the search for U.S. domestically produced, renewable transportation fuels, such as biodiesel, which is developing into a widely accepted alternative fuel. The U.S. biodiesel industry is struggling because of high prices for its feedstocks, vegetable oils, and animal fats. High feedstock prices also are affecting international biodiesel production, but requirements to lower greenhouse gas emissions continue to stimulate interest in the fuel. The authors of this new CAST Commentary explain the biodiesel production process and discuss quality requirements, characteristics of biodiesel, biodiesel economics, and energy balance.