The book, consisting of two volumes, presents a critical edition and an annotated English translation of the work on Hebrew grammar al-Kitab al-Kafi fi al-Luga al-‘Ibarniyya by the medieval Karaite grammarian 'Abu al-Faraj Harun Ibn Faraj. This was one of the most important works on Hebrew grammar that was written in the Middle Ages, which, however, was lost to knowledge for several centuries and is here recovered from medieval manuscripts for the first time in a modern edition. In addition to the text edition and translation, the book contains an introduction on the background of the text and the codicology of the manuscripts. This publication will be of interest not only to Hebraists and Biblical scholars but also to scholars concerned with the history of linguistic thought and medieval thought in general.