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.