This paper studies Korean vowel shortening (VS) in verb/adjective bases triggered by suffixes and proposes that VS in derivational morphology is in fact Multiple Exponence (ME). I argue that VS should be treated differently in inflectional and derivational morphology. In inflectional morphology, on the one hand, VS-triggering suffixes show a phonologically conditioned regular pattern. In derivational morphology, on the other hand, VS occurs when followed by a lexically specified suffix. In this case, I propose that a triggering suffix and a base modification together constitute ME. To distinguish :ME from phonologically conditioned rnorphophonological processes, I propose four criteria for :ME, examining properties of :ME studied on Matthews (1972). VS in derivational morphology is examined through these criteria, and I conclude that VS and an accompanying-suffix constitute ME in Korean.