In chemistry, a hydrocarbon is a group of chemical compounds consisting only of carbon (C) and hydrogen (H). They all consist of a carbon backbone and atoms of hydrogen attached to that backbone. (Often the term is used as a shortened form of the term aliphatic hydrocarbon.)

For example, methane (swamp gas/marsh gas) is a hydrocarbon with one carbon atom and four hydrogen atoms: CH₄. Ethane is a hydrocarbon (more specifically, an alkane) consisting of two carbon atoms held together with a single bond, each with three hydrogen atoms bonded: C₂H₆. Propane has three C atoms (C₃H₈) and so on (C_nH_2·n+2).

There are basically three types of hydrocarbons:

1. aromatic hydrocarbons, which have at least one aromatic ring in addition to whatever bonds they have
2. saturated hydrocarbons, also known as alkanes, which don't have double, triple or aromatic bonds
3. unsaturated hydrocarbons, which have one or more double or triple bonds between carbon atoms, are divided into:

• alkenes
• alkynes
• dienes

The number of hydrogen atoms in hydrocarbons can be determined, if the number of carbon atoms is known, by using these following equations:

• Alkanes: C_nH_2n+2
• Alkenes: C_nH_2n (assuming only one double bond)
• Alkenes: C_nH_2n-2 (assuming only one triple bond)

Liquid geologically-extracted hydrocarbons are referred to as petroleum (literally "rock oil") or mineral oil, while gaseous geologic hydrocarbons are referred to as natural gas. All are significant sources of fuel and raw materials as a feedstock for the production of organic chemicals and are commonly found in the subsurface using the tools of petroleum geology.

Hydrocarbons are of prime economic importance because they encompass the constituents of the major fossil fuels (coal,petroleum, natural gas, etc.) and biofuels, as well as plastics, waxes, solvents and oils. In urban pollution, these components--along with NOx and sunlight--all contribute to the formation of tropospheric ozone.