Fossil fuels are fossils in the sense of being derived from buried organic material. As with virtually all living material, the central element is carbon and this is the essential component of all of the fuels. The biological matter that decomposes in the process of its conversion to fossil fuel does so under anoxic conditions, buried at various depths and prevented from oxidation by isolation from the atmosphere. The carbon compounds contain a variety of elements, of which all but one are incidental to fuel generation. That one is hydrogen, and it is a sufficiently large fraction of the decomposing material to become an important, even a major, ingredient of most fuels. These are basically hydrocarbons with proportions of H/C varying from almost zero for the highest rank of coal to 4:1 (atomic ratio) for methane, CH4, the lightest hydrocarbon, which is the dominant constituent of oil-related natural gas as well as the ‘unconventional’ coal seam gas and shale gas. The formation and classification of the three fuel types, coal, oil and gas, are considered in Sections 24.2 to 24.4. Data on fuel production and reserves are presented in Sections 24.5 and 24.6. Section 24.7 lists the principal uses and the CO2 released by combustion. Inevitably, some specialist jargon has developed, especially in the oil industry, and is referred to in Section 24.8.