Infants and children are more vulnerable to interference from competing backgrounds sounds than adults. These development effects in auditory masking can be substantial for children with normal hearing sensitivity, but are more pronounced for children with hearing impairment. Despite the fact that infants must learn about speech and language in the presence of competing sounds, we have a limited understanding of the factors that influence children's hearing in noise, and few studies have addressed the specific challenges faced by infants and children with hearing impairment. The long-term aim of this project is to identify and explain the factors responsible for the development of hearing in complex acoustic environments that contain multiple sources of sound. The proposed experiments will characterize developmental effects in the ability to hear target tones or speech in the presence of competing background sounds and will identify acoustic-cue combinations that improve hearing in noise for both normal-hearing and hearing-impaired infants and children. Aim 1 will delineate the time course of development for remote-frequency masking from infancy to adolescence. The proposed studies will test the hypothesis that susceptibility to interference from remote-frequency background sounds decreases with increasing age. Aim 2 will investigate the extent to which infants and children benefit from the provision of robust and/or redundant auditory grouping cues in the presence of competing background sounds. These studies will test the hypothesis that infants and children benefit less than adults from sound source segregation cues when those cues are relatively sparse. Aim 3 will identify acoustic cues that improve hearing in the presence of competing background sounds for infants and children with hearing impairment. This work will test the hypothesis that hearing impairment interferes with auditory processing abilities related to the segregation and selection of target from background sounds. Experiments proposed for all three aims rely on rigorous behavioral methods using well-characterized multi-tonal and noise stimuli as well as speech perception testing in the presence of competing noise or speech maskers. The results of the proposed studies will provide much needed normative data and are expected to contribute to the formation of pediatric measures of complex auditory perception. There is also the potential these data will lead to improved strategies to optimize the delivery of auditory information for infants and children with hearing impairment.