Most speech conversations occur in the presence of competing sounds and acoustic reflections from room surfaces. Hearing-impaired people often complain of difficulties understanding speech in such complex acoustic environments even if they do well in quiet. Here we propose neurophysiological and computational studies that address fundamental questions about two aspects of listening in complex environments: (1) How the auditory system compensates for the degradation in the acoustic signal caused by reverberation;(2) How the auditory system extracts the pitch of harmonic complex tones, one of the main cues used by listeners to segregate simultaneous sound sources. We will record from single units in the auditory nerve (AN), ventral cochlear nucleus (VCN) and inferior colliculus (IC) in response to complex sounds that incorporate some features of complex acoustic environments, and develop computational models that predict these responses. Specific Aim 1 is to test the hypothesis that the auditory system contains neural mechanisms that allow it to preserve good directional and temporal sensitivity in reverberation. We will measure the directional and temporal envelope sensitivity of IC neurons in simulated room environments, compare these responses with predictions of existing models of binaural processing, and develop new models incorporating reverberation compensation mechanisms. Aim 2 is to test the hypothesis that the cochlear traveling wave creates robust spatio-temporal cues to the pitch of complex tones that can be extracted by a neural mechanism sensitive to the relative timing of spike discharges from AN fibers tuned to slightly different frequencies. We will test the availability and robustness of these spatio-temporal pitch cues in the AN, then examine whether these cues can be extracted by neurons in the VCN known to be sensitive to monaural phase. This research addresses fundamental issues in auditory theory such as the neural mechanisms for pitch processing, the mechanisms for echo suppression in reverberation, and mechanisms of sound source segregation. It may lead to a better understanding of why hearing-impaired and elderly listeners have greater difficulties understanding speech in the presence of reverberation and competing sounds than do normal listeners, and may help develop new kinds of hearing aids and auditory (cochlear and brainstem) implants that perform better in challenging environments.