The term fuel refers to any substance which is burned to produce energy, typically for heating, cooking, or producing mechanical work in a motor.
- Firewood is the oldest fuel used by humans, and is still an important fuel source for home heating and for cooking in developing countries. Wood gas and charcoal are derivates of wood also used for extracting energy.
- Coal was the most important fuel during the 19th century, and continues to be widely used in China and in the steelmaking industry.
- Petroleum was the most important fuel during the 20th century. The transition from a petroleum-based economy to a post-petroleum economy is perhaps the most important issue facing the world today.
- Biofuels including natural gas, ethanol, biomethanol and vegetable oils (from land-based and sea/pond-based plants) are widely used for home heating, cooking, industrial applications, and large-scale electricity generation.
- Hydrogen, oxyhydrogen, nitrous oxide, syngas, compressed air and other zero-emission gases are currently on the rise. Most of these (hydrogen, oxyhydrogen, syngas ) have high amounts of energy density, but storage of the gases is still very problematic. Nitrous oxide has high amounts of energy density, and is easy to store yet can't be burned that easily. Compressed air is the only gas that has but little energy density per m³ of gas.
- Nuclear fuel is another source of (zero-emissions) fuel