Multilingualism is on the rise – in the coming decades, as many as two billion people will learn English as a second language. The next stage up from multilingualism is the domain of the 'hyperpolyglot' or 'superlearner': someone who claims to know at least six languages. But what does it mean to 'know' a language? Can a person claim to speak a language fluently if it isn't their mother tongue? What role does culture play in learning languages? In this accessible and enthralling book, Michael Erard discusses the upper limits of the brain's capacity to learn languages and sheds light on the 'hyperpolyglot' phenomenon, from the Italian cardinal Giuseppe Mezzofanti, who was said to speak as many as seventy-two languages, to the 'superlearners' of the 21st century.