Being bilingual is a common experience for many people. In this chapter we define what is bilingualism according to the most recent research evidence in linguistics and neurolinguistics. A general overview of the different stages of bilingualism across the lifespan and the terminology are presented together with preliminary insight on the properties and capacities of the bilingual brain. Findings reported in the chapter include the role of minority languages, such as local and regional languages spoken by small communities, and their crucial role for sustaining bilingualism globally.

This chapter focuses on one of the most remarkable characteristics of bilinguals, namely their ability to effortlessly switch between two languages, and to combine grammar rules and words from each in one sentence. It offers a summary of what we know about the ways in which bilingual children code-switch and how this skill develops over time. Attention is paid to the variability in the patterns found in data sets from typologically different languages and social contexts. Analyses are also offered of the relationship between code-switching and language proficiency or language dominance. Differences or similarities with respect to switching among bilingual and trilingual children are discussed as well. Particular attention is given to the difficult issue of the (lack of) separation of grammars in code-switching, and to what extent code-switching patterns found among children differ from those of adults. Finally, there is a focus on the impact of code-switching on cognitive control. The chapter finishes with a summary of the key points and an outlook towards the future.