Acoustical design and computer modeling rely on the availability, accuracy, and use of coefficients which characterize how boundary surfaces affect incident sound. At this time three coefficients have been developed to characterize how incident sound is modified by absorptive and scattering surfaces. The random incidence absorption coefficient, which is a measure of the proportion of incident sound that is absorbed, is measured according to ISO 354/ASTM C423. It is well known that coefficients measured according to this standard suffer from several systematic errors which will be reviewed. Measurement of the random incidence scattering coefficient, which is the proportion of incident energy scattered in non-specular directions and used in computer modeling programs, is experimentally determined by ISO 17497-1. The limitations of this standard and applications will be discussed. Measurement of the diffusion coefficient, which is a measure of the uniformity of scattered sound and used to evaluate potential surfaces and in optimization programs, is described in AES-4id-2001 and ISO 17497-2 (pending). As with all standards it too has limitations which will be discussed. Detailed information on all three coefficients is given in Cox, T. J., and DAntonio, P. (2009). Acoustic Absorbers and Diffusers (Taylor & Francis, London).