Code-switching - the alternating use of several languages by bilingual speakers - does not usually indicate lack of competence on the part of the speaker in any of the languages concerned, but results from complex bilingual skills. The reasons why people switch their codes are as varied as the directions from which linguists approach this issue, and raise many sociological, psychological, and grammatical questions. This volume of essays by leading scholars brings together the main strands of current research in four major areas: the policy implications of code-switching in specific institutional and community settings; the perspective of social theory on code-switching as a form of speech behaviour in particular social contexts; the grammatical analysis of code-switching, including the factors that constrain switching even within a sentence; and the implications of code-switching in bilingual processing and development.