Guion, Susan G. & Lee, Borim. 2006. The Role of Phonetic Processing in Second Language Acquisition. English Language and Linguistics 21, 123-148. Experience with a first language shapes the processing of phonetic input. Acoustic information that is used to cue phonological distinctions in the first language is weighted more heavily. Correspondingly, acoustic information not used to cue phonological distinctions comes to be systematically under-attended. In the domain of perception, Guion and Pederson (in press) investigated the plasticity of the attentional system with regard to processing novel acoustic cues. The acoustic information used by native speakers of a tone language, naïve speakers of non-tone languages, and adult learners of a tone language was investigated. The adult learners were found to have retuned their phonetic processing to attend to acoustic cues employed by tone languages. In the domain of production, Lee, Guion, and Harada (in press) investigated the relationship between acoustic cues used in first language prosody and the production of English unstressed reduced vowels. Korean-English and Japanese-English bilinguals who began learning English before the age of 6 or after the age of 15 were studied. The early bilingual groups were nearly native-like but showed some characteristics that could be attributed to first language processing. The late bilinguals were less native-like but, nonetheless, exhibited productions closer to native-like norms than would be predicted by the transfer of the first language phonological system alone. These results indicate that early learners can learn to attend to acoustic cues not used in the phonology of their first language and, thus, develop productions that come close to native norms. Late learners, on the other hand, show evidence of perceptual plasticity, but are less likely to attain native-like production norms than early learners.