This volume, together with its predecessor (Vol.1: Hunayn ibn Ishaq's Arabic Translation), presents the first editions of the Critical Days since 1825, and the first translation into a modern language. Furthermore, they contextualize the treatise within the Greek and Arabic traditions. Galen's Critical Days was a founding text of astrological medicine. In febrile illnesses, the critical days are the days on which an especially severe pattern of symptoms, a crisis, was likely to occur. The crisis was thought to expel the disease-producing substances from the body. If its precise timing were known, the physician could prepare the patient so that the crisis would be most beneficial. After identifying the critical days based on empirical data and showing how to use them in therapy, Galen explains the critical days by the moon's influence. The previous volume discusses the translation of the Critical Days in Arabic and its context in 9th-century Baghdad, and addresses issues of translation as well as important concept. In the present volume, the introduction places the treatise within the contexts of Galen's life and career, and within the history of the Greek medical and philosophical tradition. The significance of the treatise for the history of scientific method and astrological medicine, respectively, is assessed in detail. The Greek edition is based on all extant manuscripts, early printed editions, and textual evidence inferred from the study of the Arabic translation presented in Volume 1. The Commentary comprehensively discusses issues related to the Greek tradition. Appendices include a Graeco-Arabic Glossary of translation.