Aliphatic compounds are a class of organic compounds in which the atoms are connected by single, double, or triple bonds to form nonaromatic structures. They can be saturated (in which all the C-C bonds are single) like hexane, or unsaturated, like hexene and hexyne. Open-chain compounds, whether straight or branched, and which contain no rings of any type, are always aliphatic. Cyclic compounds can be aliphatic if they are not aromatic.

Aliphatic compounds can be divided into three types of hydrocarbons: alkanes, alkenes, and alkynes/09%3A_Organic_Chemistry/9.02%3A_Aliphatic_Hydrocarbons). Alkanes are aliphatic hydrocarbons with only single covalent bonds, while alkenes are hydrocarbons that contain at least one C–C double bond, and alkynes are hydrocarbons that contain a C–C triple bond/09%3A_Organic_Chemistry/9.02%3A_Aliphatic_Hydrocarbons). Occasionally, aliphatic hydrocarbons with a ring of C atoms are found, and these hydrocarbons are called cycloalkanes (or cycloalkenes or cycloalkynes) /09%3A_Organic_Chemistry/9.02%3A_Aliphatic_Hydrocarbons).

Aliphatic compounds are typically flammable, allowing the use of hydrocarbons as fuel, such as methane in natural gas for stoves or heating, butane in torches and lighters, and various aliphatic (as well as aromatic) hydrocarbons in liquid transportation fuels like petrol/gasoline, diesel, and jet fuel.