This book is a research monograph divided into two parts. The first part describes the methods used to build the first sizeable corpus of informal conversational data collected from bilingual speakers of Welsh and English: Siarad. The second part describes the linguistic analysis of data from this corpus (available at bangortalk.org.uk). The information in Part One will be useful as a ‘how to’ manual on building a bilingual spoken corpus, including methods of data collection, transcription, glossing and analysis. The findings reported in Part Two throw new light on the debate regarding code-switching vs. borrowing, the application of the Matrix Language Framework (MLF) to the grammar of Welsh-English code-switching, the extralinguistic factors influencing variation in quantity of code-switching, and the extent to which the grammar of Welsh is changing in contact with English. Additional findings by other researchers using the corpus are also reported, and possible future directions are discussed.