A review is given of the properties of speech sounds that identify different phonetic categories. It is hypothesized that the auditory system gives a distinctive response to sounds with these special properties and hence facilitates classification of the sounds in terms of phonetic categories. The properties that will be discussed include: presence of a rapid spectrum change; presence of an abrupt increase in amplitude; a narrow midfrequency spectral prominence as opposed to a diffuse spread of spectral energy; spectral energy concentrated at high frequencies as opposed to low frequencies; periodic as opposed to nonperiodic sound; spectral characteristics of vowels; presence of low-frequency sound preceding an onset; two onsets that are simultaneous as opposed to sequential. Evidence for these acoustic properties is presented both from acoustic analysis and from experiments on the perception of speech and speechlike sounds. [Supported by NIH grants.]