Vowel Harmony (VH), phonological co-occurrence constraints between the features of vowels in polysyllabic words, is a fossilized remnant of an earlier phonetic process involving vowel-to-vowel assimilation. An understanding of the workings and constraints of VH can give us clues on how such inter-syllabic assimilations work and would be important for spoken language processing, e.g., motivating speech recognition using triphones. A universal, phonetically-based, theory of vowel harmony should explain why only certain distinctive features of vowels are subject to harmony, how "neutral" vowels arise, what intervening phonetic contexts favor or disfavor harmony.