“On 4 November 1995 Kasabe existed; on 5 November it did not.” This brief obituary in David Crystal's book (p. 1) does not refer to a creature. It refers to an African language, known as Luo, that had been spoken in Cameroon; on the day in question, Bogon, the last speaker of Kasabe, died, taking his language with him. According to Crystal, half of the world's current languages are in danger of dying, and the situation is so grave that an international and interdisciplinary effort must be mounted to rescue them. Crystal has written this book to alert us to the peril and to suggest ways in which our linguistic diversity might be maintained.