Physical and chemical characteristics of waste streams influence the ease with which oil emulsifies in water and also the stability of the resulting emulsion. The interrelationships of these parameters and their impact on emulsion stability via imposed mixing energy is examined. Electrokinetic aspects of phase stability for several crude oil-in-water emulsions are presented as well as other conventional stability factors and their interrelationship. Photomicrograph techniques and how they can be utilized for in situ surveys of oily waste streams are presented. Use of this technique yields data to assess the efficacy of existing facilities for waste handling or treatment in terms of emulsion destabilization-coalescence efficiency, waste stream characterization, or troubleshooting of operational problems. By sampling and analysis at several stations in a system, more specific data can be generated. The applicability of photomicrograph techniques to provide rational design data for oil-water separation systems is given in several practical examples. Performance in the majority of oil removal devices depends to some degree on the driving force which exists to separate the two phases. For conventional gravity separators, this driving force is expressed in terms of Stoke's Law which incorporates oil droplet diameter. Oil removal efficiencies for gravity separation systems can be accurately estimated if the influent oil droplet size distribution is known. Examples of wastewater characterization for process wastewaters, tanker ballast, and storm-water runoff are given as well as their respective treatment system performance.