Fuels derived from the remains of plants and animals buried underground millions of years ago throughout the earth's evolution. These organic remains contain carbon and hydrogen molecules and release energy when subject to combustion. Fossil fuels include coal, crude oil, and natural gas and have been the preeminent source to meet the world's energy demand.

Fossil fuel consumption, however, releases carbon dioxide, which contributes to the greenhouse effect where heat is trapped in the earth's atmosphere. Increasing evidence of the direct link between carbon dioxide emissions and climate change, has fueled the energy transition spurring global action towards phasing-out reliance on fossil fuels and increasing the development of renewable energy sources.

For information on the oil & gas industry, see Practice Note, US Oil & Gas Industry: Overview. For information on renewable energy, see Practice Note, Renewable Energy: Overview.