If you have you ever overheard a conversation between two people who share more than one language you'll have noticed that they do not always stick to one language. Instead they constantly move from one language to the other in a seemingly random fashion. Why do they do it? Is it because they are unable to stick to one language? Is it that they are too lazy to do so? Here, as everywhere else, appearances are misleading. Speaking of bilingualism, everybody, linguist or not, seems to have an opinion about it either as a good or a bad thing, either as an advantage or as a disadvantage. Exactly what practical advantages are there in being bilingual? What do bilingual speakers do with their two or more languages in interaction with other bilinguals? This book investigates these and other related questions about the practical usefulness of bilingualism.