Using a corpus of data drawn from naturally-occurring second language conversations, this book explores the role of idiomaticity in English as a native language, and its comparative role in English as a lingua franca. Through examining how idiomaticity enables first language learners to achieve a greater degree of fluency, the book explores why idiomatic language poses such a challenge for users of English as a lingua franca. The book puts forward a new definition of competence and fluency within the context of English as a lingua franca, concluding with an analysis of practical implications for the lingua franca classroom. This in-depth study of English language learning using corpus data will be of interest to researchers in applied linguistics and corpus linguistics and to teachers of English as an international lingua franca.