From a purely thermodynamic point of view, an emulsion is an unstable system because there is a natural tendency for a liquid/liquid system to separate and reduce its interfacial area and, hence, its interfacial energy. However, most emulsions demonstrate kinetic stability (i.e., they are stable over a period of time). Produced oilfield emulsions are classified on the basis of their degree of kinetic stability. Water-in-oil emulsions are considered to be special liquid-in-liquid colloidal dispersions. Their kinetic stability is a consequence of small droplet size and the presence of an interfacial film around water droplets and is caused by stabilizing agents (or emulsifiers). These stabilizers suppress the mechanisms involved that would otherwise break down an emulsion. Sedimentation is the falling of water droplets from an emulsion because of the density difference between the oil and water. Aggregation or flocculation is the grouping together of water droplets in an emulsion without a change in surface area.