This is an introduction to Optimality Theory, whose central idea is that surface forms of language reflect resolutions of conflicts between competing constraints. A surface form is 'optimal' if it incurs the least serious violations of a set of constraints, taking into account their hierarchical ranking. Languages differ in the ranking of constraints; and any violations must be minimal. The book does not limit its empirical scope to phonological phenomena, but also contains chapters on the learnability of OT grammars; OT's implications for syntax; and other issues such as opacity. It also reviews in detail a selection of the considerable research output which OT has already produced. Exercises accompany chapters 1-7, and there are sections on further reading. Optimality Theory will be welcomed by any linguist with a basic knowledge of derivational Generative Phonology.