It has long been debated whether speech processing remains flexible in adulthood. This thesis contributes to our understanding of this question by investigating bilingual speech development in a naturalistic setting. Galician ‘new speakers’ (neofalantes) are unbalanced bilinguals raised with Spanish as a primary language, who learn Galician at an early age in a bilingual environment, but in adolescence, decide to switch to using Galician almost exclusively, for ideological reasons. Study 1 examined whether neofalantes changed aspects of their production and perception of Galician post-switch. Change was inferred by com- paring this group to two control groups, Galician-dominant and Spanish-dominant bilinguals. Results showed that neofalantes pattern with Spanish-dominants in their perception and production of mid-vowel and fricative contrasts, but with Galician-dominants in their realisation of unstressed word-final vowels, a highly salient feature of Galician. However, Study 2 demonstrated that these shifts in production were not sufficient to enable Galician listeners to identify the neofalantes’ accent as a distinctive variety. Instead, neo- falantes were categorised as both Galician- and Spanish-dominant speakers. Study 3 used eye-tracking to investigate the effects of language dominance and long-term language switch on spoken word recognition. Results showed that recognition was slower for Spanish-dominants, however, the level of lexical activation of the confusable competitors was similar for Galician- and Spanish-dominant groups. Like in perception tasks, neofalantes behaved more similarly to Spanish-dominant listeners. These results indicate that despite early exposure to Galician, high motivation and almost exclusive Galician language use post-switch, there are limitations to what neofalantes can learn in production, perception and processing. However, although underlying categories appear hard to change, with modifications to production and perception constrained by early experience with a particular language, the resulting hybrid categories may function as opportunities to mark identity within a community.