The study of code-switching has been one of the most dynamic areas in linguistics over the last three decades, at least since Poplack's influential paper on Puerto Rican Spanish-English bilingual speech in New York. The demarcation line between code-switching and bilingual interference is definitional: in the case of interference the interaction of the two languages is structural rather than involving phonetic material: words or morphemes from the two languages. This chapter presents some of the issues raised in the vast literature on code-mixing in three main sections: sociolinguistics, grammar, and language use. Code-switching capacities develop and change across the life span of an individual. The major methodological problem in the pragmatic tradition is that the interpretation of conversations in which codes are switched remains subjective. Labovian tradition of accountable analysis of naturalistic speech data is stressed in the work of Poplack and associates as in the work of MacSwan.