Kim, Sook-Hee. 2002. A Historical Approach to the Middle English Vowel /a/. The History of English, 14. This paper takes up the issue concerning how the vowel /a/ changed during the Middle English period. Compared with other short vowels, this vowel was considered unstable. This paper also examines the history of the West Germanic /a/ and other data pertaining to the motivation for the instability of the vowel. Andersen's (1973) model of abductive and deductive change is employed to understand such a linguistic change. Acoustic distinctive features are taken into consideration, rather than a more commonly-used articulatory feature. Problems and innovations, involved in the change of /a/, are reviewed and examined, in a way to specify the cause and effect of the change. This paper compares and studies the occurrence of /a/ for the Middle English /a/ and /æ/ and shortened /æa/, which appeared at the beginning of the 12th century. This paper also handles the historical changes /y/>/i/ and /ø/>/e/ with regard to the unrounding of front rounded vowels, while, on the other hand, examining the historical process of the merger of /æ/ and /a/ into /a/. To show the several degrees of vowel height in other languages as a piece of cross-linguistic data, this paper looks more closely into a Bulgarian phonological aspect and touches on the relatedness between tenseness and vowel dimension.