Fuels are any materials that store potential energy in forms that can be practicably released and used for work or as heat energy. The concept originally applied solely to those materials storing energy in the form of chemical energy that could be released throughcombustion,^[1] but the concept has since been also applied to other sources of heat energy such as nuclear energy (via nuclear fission ornuclear fusion).

The heat energy released by many fuels is harnessed into mechanical energy via an engine. Other times the heat itself is valued for warmth, cooking, or industrial processes, as well as the illumination that comes with combustion. Fuels are also used in the cells oforganisms in a process known as cellular respiration, where organic molecules are oxidized to release un-usable energy. Hydrocarbonsare by far the most common source of fuel used by humans, but other substances, including radioactive metals, are also utilized.

Fuels are contrasted with other methods of storing potential energy, such as those that directly release electrical energy (such as batteriesand capacitors) or mechanical energy (such as flywheels, springs, compressed air, or water in a reservoir).