Chapter 7 deals with a longstanding controversy from both historical and theoretical linguistic perspectives. The chapter provides a description of Korean vowel harmony beginning with an overview of its historical and theoretical linguistic background. Middle Korean displays relatively rigorous patterns of vowel harmony; contemporary varieties show reduced but still complicated patterns that are confined to certain morphological contexts, as a result of historical changes. The description of Korean vowel harmony in this chapter is divided mainly into two parts: the relatively complete vowel harmony system in Middle Korean, and vowel harmony in contemporary (chiefly Seoul) Korean. The former provides an historical and typological overview of the productive MK patterns, while the latter focuses on the exceptions and variations of the remnant harmony patterns in ideophones and verbal inflections. The remainder of the chapter introduces a number of theoretical approaches, from early generative treatments to more recent Optimality Theoretic analyses.