Acquiring one's native language phoneme categories alters perception so that individuals become more sensitive to between- than within-category acoustic variation. Although this pattern of perceptual warping is adaptive for understanding one's native language, perceptual warping can also emerge from hearing and language disorders. This talk will discuss how these kinds of perceptual warpings can interfere with learning a new language as an adult, and interfere with the ability to make other adjustments to phonetic perception (e.g., when acclimitizing to a cochlear implant). I will give examples from recent work on the acquisition of English /r/-/l/ by Japanese adults, the perception of English /w/-/v/ by native Sinhalese speakers, and nativelanguage speech perception by adult cochlear implant patients. The results suggest that at least some aspects of the speech recognition difficulties of second-language learners and adult cochlear implant patients can be explained within the same theoretical model.