"Paradigms and Opacity" integrates Optimality Theory and Lexical Phonology, presenting a stratal OT model in which the properties of stems, words, and sentences are characterized by distinct parallel constraint systems that interface serially. This model accounts for morphology/phonology interactions and opaque constraint interactions better than models found in current versions of OT phonology. Evidence is provided from a very broad range of languages, and further support is offered on grounds of learnability, naturalness, historical change, and typological restrictiveness.