This paper reviews the use of vegetable oils and animal fats as diesel feul. Physical and chemical properties and structure of the vegetable oils are discussed. Fuel preparation by transesterification, pryolysis, dilution, and microemulsion and the effects of these processes on the properties of the fuel and, in turn, their effects on the engines have been reviewed. Each of the processes give improved fuel properties over those of unprocessed vegetable oil. The performance of engines using triglyceride based fuels and their emission characteristics are also presented.