This book of a comprehensive study that explores the Matter of Arab in Chaucer; the Matter which is totally neglected by Chaucerian scholars who acknowledged three Matters only: that of France, of Rome and of Britain. By the Matter, it is meant all materials (vocabulary- nouns, verbs ...etc, plots, characters, settings, motifs, themes, narrative links and structure and frame-stories) that were directly or indirectly borrowed by Chaucer from a main big storehouse of a related domain. The study asseverates the unfathomed role of the Medieval Arabs advanced learning and life as well as their literary works in making Chaucer; the role that worthes to be handled as a Matter of Arab. Here lies the significance of this study. The book traces the Arabic lexicons (or loans) in Chaucer as the first aspect of the Matter of Arab. Through statistically surveying Chaucer's Complete Works (55 in number), the research finds out that these loans are not only more than nine hundreds -not 24 as it was believed- and refer to subjects related to Arabs (their religion, celebrities, sciences, life and philosophy) or even unrelated to them, but also extend to include all Chaucer's oeuvre : the early juvenilia as well as the late works and to be indispensably used as Chaucer's language treasures. It suggests that these loans could have infiltrated the works via (1) His visit to Spain, (2) his readings in Arabic references in rendition, (3) the spread of the Arabian culture into Europe through the agencies of (a) the medieval furor of Translation, (b) the spread of manuscripts, (c) the influence of Dante, Boccaccio, Aquinas ...etc, (d) the role of the Jews in Andalusia as translators, (e) the Arabs' religious tolerance and (f) the impacts of universities, commerce, pilgrimages and crusades. Hence, Chaucer took a lot of things and above all loans from Arabian sources in rendition that were within his reach as he developed in art and experience. This is why the the Matter of Arab, as opposed to the three Matters is an overall phenomenon in Chaucer's life and extends to and within other Matters. This book is divided into: (I) Preliminaries, (II) The Direct Contact with Arabs, (III) The Indirect Contact with Arabs, (IV) Arabian Culture in the Air and (V) Findings and Conclusions and Four Appendices.