A major question in the study of second language learning is the extent to which mispronunciations originate in the inability to correctly perceive vs. the inability to correctly produce foreign language structures. The goal of this thesis is to determine the extent to which second language (L2) learners' pronunciation errors reflect errors in perception or gestural mistiming, by investigating Korean L2 learners' production and perception of English stop-nasal sequences. Such sequences are prohibited in Korean, where a stop before a nasal obligatorily undergoes nasalization (/kukmul/-->[kuŋmul] `soup'). In production experiments where Korean L2 learners pronounced English nonsense words containing those sequences, vowel insertion after the stop and devoicing of the stop were common errors even though nasalization is the native repair strategy. More importantly, two asymmetries in the choice of repair were that (1) vowel insertion occurred almost exclusively after voiced stops, especially after velar stops (tegnal-->tegVnal) and (2) devoicing occurred most frequently with labial stops (tebnal-->tepnal). These asymmetric repair choices are puzzling because neither of the languages in contact provides evidence for such repairs. Investigation of Korean speakers' perception of these sequences employing both behavioral tasks and EEG revealed that the greater frequency of vowel insertion after voiced stops was rooted in misperception: Korean listeners tended to hear an illusory vowel after voiced stops and had difficulty distinguishing voiced stop-nasal sequences from voiced stop-vowel-nasal sequences. This misperception is an effect of the native language system, in which voiced stops occur only preceding a vowel. In contrast, the frequent devoicing of labial consonants was not reflected in perception. I propose that instead, this pattern has its origin in the articulatory timing patterns of Korean, in which bilabial stops are much more closely overlapped with a following consonant than are velar stops, causing devoicing of [b]. Results of this project show that second language phonology involves a complex interplay between the native language grammar, misperception through the filter of L1, and mastery of new articulatory programs.