The term “emulsion stability” refers to the ability of an emulsion to resist changes in its properties over time: the more stable the emulsion, the more slowly its properties change. An emulsion may become unstable due to a number of different types of physical and chemical processes.* Physical instability results in an alteration in the spatial distribution or structural organization of the molecules, whereas chemical instability results in an alteration in the kind of molecules present. Creaming, flocculation, coalescence, partial coalescence, phase inversion, and Ostwald ripening are examples of physical instability (Dickinson and Stainsby, 1982; Dickinson, 1992; Walstra, 1996a, 2003a), whereas oxidation and hydrolysis are common examples of chemical instability (Fennema, 1996; McClements and Decker, 2000). The development of an effective strategy to prevent undesirable changes in the properties of a particular food emulsion depends on the dominant physicochemical mechanism(s) responsible for the changes. In practice, two or more of these mechanisms may operate in concert. It is therefore important for food scientists to identify the relative importance of each mechanism, the relationship between them, and the factors that influence them, so that effective means of controlling the stability and physicochemical properties of emulsions can be established.