Emulsions of oil droplets in water but more typically water droplets in oil are common in the production, transportation, and refining of petroleum and related products. Emulsions of water in petroleum or petroleum-derived liquids can be stabilized by a variety of surface-active compounds and components present in petroleum and also with which petroleumn comes into contact. These include asphaltenes, carboxylic organic acids of various types, fine inorganic particles, and combinations of these three types of materials. Several of these means of emulsion and thin-film stabilization of emulsions in petroleum-water mixtures are reviewed here, as well as some of the key physical and chemical means of characterizing both the materials that stabilize the emulsions and the means of characterizing the oil-water interface and the emulsion films themselves. Some of the key unanswered questions surrounding the study of emulsion stability in petroleum systems are also reviewed.