Medieval Judeo-Arabic (MWJA) was written with Hebrew characters, and used for the Judeo-Arabic literature shared by all Jewish scholars in the domain of medieval Arab-Muslim culture. Its status was like that of literary Classical Arabic among the Muslim Arabic speakers. However, MWJA had never been a living spoken language and its life did not extend beyond four or five hundred years (tenth-fifteenth centuries). Yet, Arabic continued to function as a spoken language. Its numerous dialects also served as a written communicative vehicle, and for literature in various genres. This is true in regard to medieval Judeo-Arabic, opposed to the notion that MWJA of the school of Sa'adya was the only one used by Jews in the Middle Ages. Actually, colloquial Judeo-Arabic has existed as a written language for almost fifteen hundred years, since pre-Islamic time. Today, one of the important assignments is to carry out a meticulous and comprehensive comparative examination of the ancient and later non-classical Arabic languages in order to better understand the history of Judeo-Arabic.