This is an introduction to Optimality Theory, the central idea of which 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. Optimality Theory will be welcomed by any linguist with a basic knowledge of derivational Generative Phonology.