Effects of distorting auditory feedback on acoustic manifestations of phonetic targets in children and adults are investigated. Experiment 1 measures speech degradation in an 11-year-old following sudden-onset deafness caused by cochlear implant failure. Experiment 2 measures speech changes over a year, consequent to multichannel electrical stimulation, in the same subject, and in two postlingually and two prelingually deafened children, and a postlingually deafened adult. Experiment 3 replicates and extends measures of speech changes in normal-hearing adults during wideband noise-masked auditory feedback. Experiment 4 high-pass filters the noise and narrow-band masks it in the regions of the fundamental and first, second, and third formant frequencies. Experiment 5 band-pass filters auditory feedback in the same frequency ranges as the masks of Experiment 4. Experiment 6 replicates Experiments 3 - 5 in normally-hearing children, to determine if responses to auditory feedback distortion change developmentally in the normally-hearing children, to determine if responses to auditory feedback distortion change developmentally in the normally- hearing. The results will address: 1) the existence of a critical period for speech production; 2) the effects of loss and distortion of auditory feedback on speech production; 3) the link between perception and production in acquiring speech; 4) the normal range of values for phonetic features in noisy environments; 5) how various cochlear implants affect the speech of their users with consequences for patient disposition; and 6) rehabilitation of the profoundly deaf with specific recommendations for speech therapies for cochlear implant candidates and users.