This dissertation deals with a pattern of variation which results from a historical change in progress in Korean. In an earlier stage of the language, there was a consistent process of vowel harmony which resulted in alternations between [a] and [ʌ] in verbal suffixes. In present day Korean, however, the use of suffix forms containing [ʌ] is often generalized to contexts that originally took [a]. I present results from a Google-driven corpus study, a judgment survey, a production experiment, and a spontaneous speech study showing that the likelihood of a disharmonic form is affected by three factors: the morphophonological class of the stem, the identity and position of the suffix, and the quality of the stem vowel. First, p-irregular stems generally take [ʌ]-suffix forms irrespective of the stem vowels. Second, a sentence-ending suffix -a/ʌ, frequently used in casual speech, is realized as [ʌ] even with stems containing /a/, while other harmonizing suffixes usually surface as [a]-forms in harmony with the stem vowel /a/. Third, of the two kinds of [RTR] stems (/a/-stems and /o/-stems), which originally triggered [a]-forms, /a/-stems are much more likely to allow the variation (Hong 2008). The dissertation addresses the question of why these factors should be associated with the innovative forms. First, I argue that the extension of the [ʌ]-forms to a class of irregular stems reflects changes in the subgrammar of p-irregular stems. Second, I argue that the extension of the disharmonic form to a suffix in sentence-final position is due to the fact that this position imposes more stringent faithfulness requirements. Finally, I argue that the harmony is more likely to be maintained where it facilitates lexical retrieval of the stem. The fact that harmonic suffix forms are more likely to be used with stems containing the vowel [o] than the vowel [a] reflects Korean speakers' ability to correctly identify the two vowels. I present evidence that the perception of [a] is robust, while the perception of [o] is less accurate, arguing that /o/-stems may need harmonized suffixes to enhance their perceptibility (Kaun 1995).