An emulsifier is a substance that stabilizes an emulsion by reducing the oil-water interface tension. Emulsifiers are a part of a broader group of compounds known as surfactants, or "surface-active agents". Surfactants are compounds that are typically amphiphilic, meaning they have a polar or hydrophilic (i.e., water-soluble) part and a non-polar (i.e., hydrophobic or lipophilic) part. Emulsifiers that are more soluble in water (and, conversely, less soluble in oil) will generally form oil-in-water emulsions, while emulsifiers that are more soluble in oil will form water-in-oil emulsions.
Examples of food emulsifiers are:
* Egg yolk – in which the main emulsifying and thickening agent is lecithin.
* Mustard – where a variety of chemicals in the mucilage surrounding the seed hull act as emulsifiers
* Soy lecithin is another emulsifier and thickener
* Pickering stabilization – uses particles under certain circumstances
* Mono- and diglycerides – a common emulsifier found in many food products (coffee creamers, ice creams, spreads, breads, cakes)
* Sodium stearoyl lactylate
* DATEM (diacetyl tartaric acid esters of mono- and diglycerides) – an emulsifier used primarily in baking 
* Proteins – those with both hydrophilic and hydrophobic regions, e.g. sodium caseinate, as in meltable cheese product
In food emulsions, the type of emulsifier greatly affects how emulsions are structured in the stomach and how accessible the oil is for gastric lipases, thereby influencing how fast emulsions are digested and trigger a satiety inducing hormone response.
Detergents are another class of surfactant, and will interact physically with both oil and water, thus stabilizing the interface between the oil and water droplets in suspension. This principle is exploited in soap, to remove grease for the purpose of cleaning. Many different emulsifiers are used in pharmacy to prepare emulsions such as creams and lotions. Common examples include emulsifying wax, polysorbate 20, and ceteareth 20.
Sometimes the inner phase itself can act as an emulsifier, and the result is a nanoemulsion, where the inner state disperses into "nano-size" droplets within the outer phase. A well-known example of this phenomenon, the "ouzo effect", happens when water is poured into a strong alcoholic anise-based beverage, such as ouzo, pastis, absinthe, arak, or raki. The anisolic compounds, which are soluble in ethanol, then form nano-size droplets and emulsify within the water. The resulting color of the drink is opaque and milky white.