The recent global increase in demands for petroleum oil is leading to continuous price inflation that strongly impacts the economy. Diesel is one of the fuel sources that play a vital rule in many of our basic needs. However, its widespread use along with other conventional sources of energy is exhausting the tight budget of our countries and is greatly contributing to air pollution and global warming problems. This book, therefore, provides one of the best economic and environmental-friendly alternative sources of diesel fuel. The biodiesel derived from vegetable oils probably is the best alternative fuel that has similar combustion properties to those of ordinary petro-diesel fuel. Fresh and waste frying vegetable oils were collected and characterized. Nine various blends of vegetable oils and diesel were prepared. The important chemical and physical properties of these blends were evaluated and compared to that of diesel. The evaluations should promote other fields of research in order to meet our local and global needs. It can also contribute to solve problems in both energy and environment sectors in many different ways.