The book serves as a 'how to' guide for other researchers to emulate when building a linguistic corpus, including advice on finding and recording participants, on coding the data and on analysing the data. It contains a detailed description of our innovative auto-glossing system, which reduces the need for laborious manual grammatical glossing of language data. The book also describes findings on research in Welsh linguistics and bilingualism, including on variation in code-switching in the speech of Welsh-English bilinguals and on what the corpus can tell us about language change in Welsh. As a minority language that has endured despite long-standing pressure from the English language, research on Welsh is important and valuable as a way to understand what happens (grammatically and socially) to languages which are in intense contact with a majority language. Those who are interested in bilingualism beyond Wales will be able to extrapolate the findings to other similar linguistic situations, particularly those of minority languages.