Optimality Theory substitutes constraints for rules in universal grammar and linguistic performance. Constraints are ranked so that a lower-ranked constraint may be violated in order to satisfy a higher. OT has revolutionized phonological theory, and its insights are now being applied to other central aspects of language. This book presents the first fruits of such research as applied to syntax and to language acquisition, as well as considering the main lines of attack on OT by rule-based grammarians (for example, that constraints are ad hoc, limitless, or themselves contain rules).