Over half of the world's population is bilingual and yet few of us understand how this complex ability really works. How do two languages co-exist in the same brain? What are the advantages and challenges of being bilingual? How do we learn - and forget - a language? In the first study of its kind, leading expert Albert Costa shares 20 years of experience to explore the science of language. The definitive study of bilingualism and the human brain from a leading neuropsychologist.