A surfactant is a chemical compound that decreases the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word surfactant is a blend of "surface-active agent", coined in 1950. As they consist of a water-repellent and a water-attracting part, they are emulsifiers, enabling water and oil to mix. They can also form foam, and facilitate the detachment of dirt.

Surfactants are among the most widespread and commercially important chemicals. Private households as well as many industries use them in large quantities as detergents and cleaning agents, but also as emulsifiers, wetting agents, foaming agents, antistatic additives, and dispersants.

Surfactants occur naturally in traditional plant-based detergents, e.g. horse chestnuts or soap nuts; they can also be found in the secretions of some caterpillars. Some of the most commonly used anionic surfactants, linear alkylbenzene sulfates (LAS), are produced from petroleum products. However, surfactants are increasingly produced in whole or in part from renewable biomass, like sugar, fatty alcohol from vegetable oils, by-products of biofuel production, and other biogenic material.

Surfactants are compounds with hydrophilic "heads" and hydrophobic "tails." The "heads" of surfactants are polar and may or may not carry an electrical charge. The "tails" of most surfactants are fairly similar, often consisting of a hydrocarbon chain (linear or branched) and may comprise aromatic units. Most commonly, surfactants are classified according to the polarity of their head group: A non-ionic surfactant has no charged groups in its head. The head of an ionic surfactant carries a net positive, or negative, charge. If the charge is negative, the surfactant is more specifically called anionic; if the charge is positive, it is called cationic. If a surfactant contains a head with two oppositely charged groups, it is termed zwitterionic, or amphoteric.

However, surfactants may also be classified based on chemical structure or based on their properties / their application.

Anionic surfactants contain anionic functional groups at their head, such as sulfate, sulfonate, phosphate, and carboxylates. Prominent alkyl sulfates include ammonium lauryl sulfate, sodium lauryl sulfate (sodium dodecyl sulfate, SLS, or SDS), and the related alkyl-ether sulfates sodium laureth sulfate (sodium lauryl ether sulfate or SLES), and sodium myreth sulfate.

Others include:

Carboxylates are the most common surfactants and comprise the carboxylate salts (soaps), such as sodium stearate. More specialized species include sodium lauroyl sarcosinate and carboxylate-based fluorosurfactants such as perfluorononanoate, perfluorooctanoate (PFOA or PFO).