The present volume offers the first critical edition of the medieval Arabic translation of Galen's Commentary on Book 6 of the Hippocratic Epidemics produced by Ḥunayn ibn Isḥāq (d. ca. 870). The edition is based on all extant Arabic textual witnesses, including the Arabic secondary transmission. Only about two thirds of the Greek original of this text is extant; the Arabic translation is therefore the only complete witness to this important work. The number and extent of quotations from this commentary in medieval Arabic medical writings, which are documented in the introduction to the volume, demonstrate that it became a crucial source for the development of medicine in the Islamic world. It also gave rise to a wide range of didactic writings which illustrate its importance for medical teaching. The English translation aims to convey some of the flavour of the Arabic text. The volume also contains comprehensive indices that map out the terminology and style of the translation.