The various factors that determine the stability and separation rate of oil from oily water mixtures are reviewed. The effect of droplet size, viscosity and unique interfacial properties promoted by the presence of surface-active agents are considered. The appropriate means to handle the separation/removal of oil from waste water effluents, as influenced by these factors, will be discussed. Wherever possible, first principles are applied; conversely, other areas are indicated where the nature of the problem dictates the reliance on empirical results and test data. Specific problems, properties and equipment are used throughout the presentation to illustrate the various concepts.Presented at the 24th ASLE Annual Meeting in Philadelphia, May 5–9, 1969.