This paper reviews the research that has been reported up to the end of 1991 on the stability of liquid fuels with a particular emphasis on shale oils. The problem of shale oil instability is an important consideration in the development of shale oil as an alternative fuel source. Instability effects on sediment, peroxide, color, and surface deposit formation are reviewed. The nature of the degradation products, the mechanisms by which they are formed, the specific compounds responsible for instability, and their control and prevention are discussed. In addition, a comparison of the stability of shale oils with other fuels is made.