For the first time in over thirty years a revolution is happening in phonology, with the advent of constraint-based approaches which directly oppose the rule-and-derivation tradition of mainstream Generative Phonology. The success of Optimality Theory and the rapidity of its spread since its official launch in 1993 is remarkable even by the general standards of post-1950s linguistics. Many phonologists appear to have been caught up in the whirlwind, as witnessed by the substance of many current working papers and conferences the world over, and the recent contents of well-established journals. Two questions naturally arise: What is Optimality Theory about? In what way is Optimality Theory superior to traditional theory, if indeed it is? In this book, leading specialists and active researchers address these issues directly, and focus deliberately on the evaluation of the two competing approaches rather than on simple displays of their applicability to limited bodies of data.