The use of two or more languages is common in most of the world. Yet, until recently, bilingualism was considered to be a complicating factor for language processing, cognition, and the brain. The past 20 years have witnessed an upsurge of research on bilingualism to examine language acquisition and processing, their cognitive and neural bases, and the consequences that bilingualism holds for cognition and the brain over the life span. Contrary to the view that bilingualism complicates the language system, this new research demonstrates that all of the languages that are known and used become part of the same language system. The interactions that arise when two languages are in play have consequences for the mind and the brain and, indeed, for language processing itself, but those consequences are not additive. Thus, bilingualism helps reveal the fundamental architecture and mechanisms of language processing that are otherwise hidden in monolingual speakers.