Emulsions are divided into dilute (volume of the dispersed phase is less than 1%), concentrated (tens of %) and high concentration ones (foam, up to 99%, for example, many ointments, creams). In concentrated emulsions, the average droplet size varies from a few microns to tens of microns, and in dilute emulsions, tens and hundreds of nanometres.

Emulsification (the process to make emulsions) takes place when the dispersed phase isolates due to condensation, or when one liquid phase is dispersed in another. Dispersion can occur spontaneously or as a result of mechanical action. Mechanical emulsification requires the use of homogenizers and dispersing agents (see dispersion). The liquids should both be normally immiscible to form an emulsion. The stability of emulsions is higher if special substances, emulsifiers, are added.

Emulsions are wide-spread in nature: milk (fat droplets in water stabilised with mixtures of proteins, lipoproteins and phospholipids), latex, crude oil emulsion. Blood is close to emulsions, as well as systems containing liposomes and microorganisms.