What does it mean to know a language well and to use it successfully? Answers to this question very considerably, both historically and culturally, and between individuals. They, also, as we are beginning to see, depend on the perspective of the person who is being asked the question: for example, an academic linguist, a language user, or a language learner. Does knowing a language imply native-speaker insight and fluency? An ability to produce sentences with no grammatical errors? The capacity to write elegantly and expressively? Being able to get your meaning across and do what you need to do? Or is it a combination of these? Which answer we choose will profoundly affect how we address any problem in which language is implicated.