This chapter is concerned with masking, or how sensitivity for one sound is affected by the presence of another sound, and also with psychoacoustic phenomena that are for one reason or another typically associated with masking. The basic masking experiment is really quite straightforward. First, the unmasked threshold of the test stimulus is determined and recorded. This unmasked threshold becomes the baseline. Partial masking can be understood in terms of a simple experiment in which the listener is presented with a tone under two different stimulus conditions. The masking produced by a particular sound is largely dependent upon its intensity and spectrum. The hapter begins with pure tones, which have the narrowest spectra. Masking, then, is not necessarily a symmetrical phenomenon. This spread of masking to frequencies higher than that of the masker has been repeatedly demonstrated for tonal maskers. The actual amount of masking may be obtained by subtracting the unmasked threshold from the masked threshold.