This volume deals with medieval comparative Semitic philology (Hebrew/Aramaic/Arabic) as practised by Hebrew philologists in the Arabic speaking lands, from Iraq to Spain, discussing its development through the generations, its technics and its theoretical basis. This research is based upon an analysis of over ten thousand occurrences of comparisons in linguistic works, biblical commentaries and the like, made by fourteen Hebrew scholars from the 10th-12th centuries CE, among them Sa?adiah Gaon, Judah b. Quraysh, David b. Abraham Alfasi, Jonah b. Janah and Isaac b. Barun. Several aspects of this comparisons are presented and studied here for the first time.