The concept of oil shale, the requisites for a sedimentary rock to be considered an oil shale and the distribution and typification of the main oil shale deposits worldwide are addressed in this chapter. As the chapter forms part of a volume devoted to the contribution of organic petrology to the study of conventional and unconventional hydrocarbon resources a significant part is devoted to the petrographic characteristics of oil shales and the difficulties involved in their analysis. In particular aspects related to the quantification of organic components, and the level of maturation have been covered in detail. At present, only a few countries are commercially exploiting oil shales both as fuel to be burned and as a means to obtain shale oil. However the reserves are huge and emerging technologies such as in-situ recovery processes are being tested. The volatility of oil prices and the need to secure energy sources have been the driving force behind this re-evaluation and characterization of little known oil shale deposits in recent years, which will hopefully contribute to a revival of scientific and political interest in oil shale research.