This thesis investigates phonetic and phonological variation in the bilingual repertoire of adolescent Welsh-English bilinguals living in North Wales. It contributes to linguistic research by, firstly, providing an account of language variation in an under-studied area (N. Wales) and context (regional minority language bilingualism) and, secondly, by examining cross-linguistic variation, and the constraints on this variation, in bilingual speech. The two variables under discussion differ in how they are realised in the two languages: /l/ is thought to be heavily velarised in both languages as a result of long-term contact and phonological convergence. Variation in the production of /r/ and realisation of coda /r/ has hitherto been reported as language-specific, though frequent transfer is said to occur from Welsh to English in predominantly Welsh-speaking areas (e.g. Penhallurick 2004: 110; Wells 1982: 390).The first aim of the study is therefore to quantify claims of phonological convergence and transfer in the speech of Welsh-English bilinguals by using a variationist sociolinguistics methodology (e.g. Labov 1966), which also considers the influence of linguistic and extra-linguistic factors on variation. Particular attention is paid to differences between a majority Welsh-speaking town and a town where English is the main language. A further distinction is made between those from Welsh-speaking homes and those from English-speaking homes who have acquired Welsh through immersion education. The second aim is to make empirically-informed theoretical claims about the nature of phonological convergence and transfer, and conceptualise cross-linguistic interaction in the speech of Welsh-English bilinguals in light of existing frameworks.Data (sociolinguistic interviews and wordlists) were collected in Welsh and English from 32 Welsh-English bilinguals aged 16-18. The sample was equally stratified in terms of speaker sex, home language, and area. The two towns compared in the study are Caernarfon (N.W. Wales, where c.88% of the population speak Welsh) and Mold (N.E. Wales, where c. 20% Welsh of the population speak Welsh). The results indicate that English [ɫ] tends to be lighter than Welsh [ɫ] in word-initial onset position for females, and in word-medial intervocalic position for both males and females. The data also show linguistic influences on the realisation of [ɫ] in both languages, and differences between males and females. The realisation of coda /r/ and production of [r] and [ɾ] in English are confined to the speech of those from Welsh-speaking homes in Caernarfon. In Welsh, use of [ɹ] is widespread and is constrained by a more complex interaction between area, home language, and sex.On the basis of these findings, I conclude that features which have undergone phonological convergence due to long-term language contact may be subject to language-specific constraints when implemented phonetically. In terms of transfer, I argue for a ternary distinction between interference, transfer, and transfer which is constrained by linguistic and/or extra-linguistic factors (cf. Grosjean 2012). Finally, I suggest that Mufwene's (2001) notion of the 'feature pool' is the most succinct way of conceptualising Welsh-English transfer and differentiate between more focussed accents of English and a less-focussed variety of North Wales Welsh.