Auditory neurons typically respond to a wide range of auditory stimuli, with responses which can vary across several stimulus dimensions. This has made classification of individual auditory neurons on a functional basis somewhat difficult. Auditory neuronal responses are therefore often described by the manner in which neural populations appear to encode such stimulus dimensions as frequency, intensity, interaural differences in signal phase or intensity, and signal envelope. Of course, with most neurons encoding several dimensions, these classifications overlap extensively. Auditory neurons also vary in the rate at which they discharge spontaneously, in the absence of external stimuli. These response characteristics can vary dramatically depending upon the state of the organism. A high degree of response plasticity may be observed with changes in attention, behavior, sleep/waking state, or with anesthesia. Due to the complexity of the auditory system, the relationships between auditory neuronal responses and their underlying anatomical substrates are just beginning to be explored. [Supported by NIH/NINCDS and the Research Service of the VA.]