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.
Consumer and industrial use of fossil fuels are critical to contemporary society:
- Coal was the most important fuel during the 19th century, and continues to be widely used in China and in the steelmaking industry.
- Petroleum, or simply "oil", 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.
- Natural gas is widely used for home heating, cooking, industrial applications, and large-scale electricity generation.
Transition to a post-petroleum economy focuses on developing alternatives: by developing alternative fuels such as biodiesel or hydrogen; or by modifying applications to reduce or eliminate the need for fuels, as by using solar heating instead of a gas burner, hydroelectric instead of a gas-fired generator, etc.
The appropriate role of nuclear power in the post-fossil-fuel economy continues to be a source of great controversy.