Emulsions are dispersions of two immiscible liquids. Common emulsions are either oil in water (O/W) or water in oil (W/O) that finds a wide range of applications in foods, pharmaceuticals, and cosmetics. These systems are thermodynamically unstable and hence over time the oil and aqueous phase tend to separate out as two distinct layers. The biggest challenge associated with emulsions is its stability over time. Various forces contributes to the stability of emulsions that is discussed in details in this chapter. An unique feature of emulsions is its concentration and temperature dependent phase inversion meaning an O/W emulsion transforms to a W/O emulsion at a specific temperature or concentration.