The present volume offers the first critical edition of Book I of the medieval Arabic translation of Galen's Commentary on the Hippocratic Epidemics produced by Hunayn ibn Ishaq (d. ca. 870). The edition is based on all extant Arabic textual witnesses, including the Arabic secondary transmission. The translation of this text became a crucial source for the development of medicine in the Islamic world, especially in the nascent field of clinical medicine; the number and extent of quotations in later Arabic medical works and the wide range of didactic writings created on the basis of this translation attest to its importance. The English translation, which aims to convey some of the favour of the Arabic translation, comes with extensive notes on the differences between the Greek original and the Arabic translation. A thorough comparison between the two versions of the commentary provided important insights into the translation style and terminology of Hunayn ibn Ishaq and his associates.