The use of two or more languages is common in most countries of the world. However, until recently, bilingualism was considered as a factor that complicates the processing of speech, cognition and the brain. In the past 25 years there have been a surge in research on bilingualism, including the study, mastery and processing of languages, their cognitive and neural foundations, and the lifelong implications of bilingualism for cognition and the brain. Contrary to the belief that bilingualism complicates the language system, new research demonstrates that all known and used languages ​​become part of the same language system. The interactions that occur when using the two languages ​​have consequences for mind and the brain and indeed for language processing itself but these implications are not additive. Thus, bilingualism helps to uncover the fundamental architecture and language processing mechanisms that locates differently in monolingual speakers.