In early societies, wood or other biological fuels were the main energy source. Today in many non-industrial societies, they continue to be used widely. Biological fuels may be seen as part of a solar economy where energy is extracted from the sun in a way that makes them renewable. However industrialization requires energy sources at much higher density and these have generally been met through the use of fossil fuels such as coal, gas, or oil. In the twentieth century a number of other options such as nuclear or higher density renewable energy sources (wind power, hydro-electric power, etc.) have also been available. Nevertheless fossil fuels represent the principal source of energy for most of the industrialized world.

Fossil fuels are types of sedimentary organic materials, often loosely called bitumens, with asphalt, a solid, and petroleum, the liquid form. More correctly bitumens are sedimentary organic materials...