This paper discusses modification of acoustic signals by two‐dimensional (time/frequency) processing of the short‐time Fourier transform. This approach is suggested by time/frequency processing in the human auditory system, and is appropriate for signals such as speech and music. Time/frequency processing has two potential advantages (1) the signal processing domain is similar to the perceptual domain, so that signal modifications are related to perceived changes, and (2) visible displays of the short‐time spectrum (spectrograms) can be used as an aid in developing and evaluating specific processes. The mathematical basis for time/frequency processing is developed, and two examples are discussed removal of broadband background noise in old recordings and compression and expansion of speech for transmission through noisy channels. [Work supported by ARPA.]