This study critically reviews how Optimality Theory (OT) has dealt with the two main topics of phonology, opacity and variation. Classic OT has faced serious challenges in that opacity requires derivation and intermediate stages and that variation is gradient and non-categorical. In the first half of this paper, we examine how opacity has been handled in OT with Candidate Chains and Stratal OT. The second half reviews how weighted constraint theories have dealt with the non-categorical properties of variation. While theories of opacity are still in disagreement in accounting for the central concept of derivational stages, variation theories have made significant progress by introducing weighted constraints and probabilistic evaluation.
