It is quite commonplace for bilingual speakers to use two or more languages, dialects or varieties in the same conversation, without any apparent effort. The phenomenon, known as code-switching, has become a major focus of attention in linguistics. This concise and original study explores how, when and where code-switching occurs. Drawing on a diverse range of examples from medieval manuscripts to rap music, novels to advertisements, emails to political speeches, and above all everyday conversation, it argues that code-switching can only be properly understood if we study it from a variety of perspectives. It shows how sociolinguistic, psycholinguistic, grammatical and developmental aspects of code-switching are all interdependent, and findings in each area are crucial to others. Breaking down barriers across the discipline of linguistics, this pioneering book confronts fundamental questions about what a ‘native language’ is, and whether languages can be meaningfully studied outside of the individuals who use them.