This Aerospace Recommended Practice is intended as a guide toward standard practices for the determination of surface cleanliness that are applicable to field operation. Some of these methods can also be used to determine quality assurance that a surface has been properly prepared and maintained. The instrumental methods are: Wettability@ Surface Potential Difference (SPD)@ Ellipsometry@ and Optically Stimulated Electron Emission (OSEE). Each instrucment is described with respect to measurement techniques@ limitations@ and advantages and types of available instruments. Elementary theoretical principles and examples of the use of each instrument are also given.