In recent times, there has been a growing interest in how Celtic languages are acquired, due to ongoing efforts for minority language revitalisation through immersion education. With contributions from a team of leading scholars, this is the first volume to bring together state-of-the-art studies on language development in both children and adults learning the three most prominent Celtic languages spoken in the UK and Ireland: Scottish Gaelic, Welsh, and Irish. It focuses on how core language areas – phonology, lexicon, morphology and syntax – are acquired by different groups of learners, providing key insights into theoretical and empirical debates around bilingual language development and linguistic change more generally. The volume also covers the socio-cultural and educational context within which these languages are learnt, highlighting how these factors affect linguistic outcomes in a minority language context. It is essential reading for academic researchers and students in developmental linguistics, sociolinguistics, bilingualism, and Celtic languages.