Rule-based theory fails to make the prediction of the functional unity of phonological processes because it has no formal mechanism to express the notion of output goal of phonological rules (Kager 1999). According to Kisseberth (1970), the reoccurrence of a common output factor guiding different rules, without being explicitly stated in the rules, is called a ‘conspiracy``. These functionally related processes can be straightforwardly accounted for by OT. Vowel shortening and compensatory lengthening in Korean seem to be opposite at first glance, but we propose, in this paper, that the constraints proposed here engage in a conspiracy to obtain vowel shortening and compensatory lengthening. Rule-based theory, however, cannot explain this conspiracy effect directly in terms of the formal structure of the analysis because a rule uniquely determines the structural change in response to the structural condition. In contrast, the OT analysis succeeds in explaining this conspiracy by extracting the generalization that underlies it. In this sense, the OT account seems to provide a more satisfactory account of the two different processes in Korean: (vowel) shortening and (compensatory) lengthening.