This book presents the first edition, translation, and study of Hunayn ibn Ishaq's Arabic translation of the Galenic treatise De diebus decretoriis (kitab ayyam al-buhran 'Concerning the Critical Days'). It makes available this key text on prognosis in medicine, and at the same time throws new light on the activities of perhaps the most important translator in 9th-century Baghdad. '... great service that Cooper has done all those interested in Graeco-Arabic studies and the history of medicine by making this Arabic version available and accessible. For this, he deserves our thanks.' Journal of Islamic Studies 'Cooper's book is an important contribution to the history of science.' Aestimatio