More people in the world are bilingual than monolingual. Historically, the component disciplines that comprise the language sciences have focused almost exclusively on monolingual speakers of a single language and largely on English as the universal language. In the past decade, there has been a shift in these disciplines to acknowledge the consequences of bilingualism for characterizing language, understanding the way languages are learned and used, and identifying the consequences of negotiating life in two languages for cognitive and brain processes. Recent studies show that bilingualism confers advantages to cognitive control at all stages of life, from infancy to old age; that contrary to popular belief, being exposed to two languages from early childhood does not create confusion but instead modulates the trajectory of language development; that signed and spoken languages produce a form of bilingualism that is similar to bilingualism in two spoken languages; and that the continual activity of both languages affects brain function and structure. Despite the excitement surrounding these discoveries, we do not understand how exposure to and use of two languages creates the observed consequences for bilingual minds and brains. Addressing these questions requires a language science that is both cross-disciplinary and international. The aim of this symposium is to illustrate the most exciting of these new discoveries and to begin to consider their causal basis.