The term 'ergativity' is used to describe a grammatical pattern in which the subject of an intransitive clause is treated in the same way as the object of a transitive clause, and differently from transitive subject. It is thus complementary to the familiar pattern of accusativity. Although there isonly one ergative language (Basque) among the familiar languages of Europe, perhaps one-quarter of the world's languages show ergative properties, and therefore pose considerable difficulties for many current linguistic theories. R. M. W. Dixon here provides a full survey of morphological ergativity splits, and investigates their semantic bases. There is discussion of how an ergative system can change into an accusative one, and vice versa; of the discourse basis of ergativity; and briefly of the problems that ergative systems pose for a number of current theoretical models.