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.