Dispersions consisting of droplets of one liquid distributed throughout another immiscible liquid are termed emulsions. Emulsions can be classified into two broad groups: water-in-oil (W/O) and oil-in-water (O/W). W/O emulsions consist of water droplets dispersed in a continuum of oil phase, whereas the reverse is true of O/W emulsions; that is, oil droplets are dispersed in a continuum of water phase. A significant portion of the world’s crude oil is produced in the form of emulsions. Large quantities of emulsion-based fluids are also used for oil-well drilling, as well as for fracturing and acidizing purposes. Interest has also been shown in the use of high-viscosity emulsions to displace oil in the secondary oil recovery processes. Emulsions are also being considered as blocking agents to control the permeability of the formation. Emulsion paints have several important advantages over solvent-based paints, such as faster drying rates and little tendency to become brittle with aging.