A considerable number of ancient medical texts has not been yet edited drawing on the whole manuscript tradition. This is also the case of the treatise On Affections, a medical book traditionally transmitted as a part of the Hippocratic Corpus. This volume offers the first critical edition of On Affections that considers the whole manuscript and printed tradition. It also includes an exhaustive account of the history of the text, a translation into English and a commentary. On Affections is unique among the Hippocratic writings in that it presents itself as a medical handbook for intelligent lay readers and not for physicians. The book includes a systematic discussion of diseases, and has clear affinities with other Hippocratic texts. Furthermore, it also contains a catalogue of foods and their properties, the combination of these two topics being unparalleled in the rest of the extant treatises. References to other existing or yet-to-be-written medical books on different topics such as eye diseases, women diseases, tertian and quartan fevers and the recipe collection called On Drugs hint at the wide circulation and availability of written medical knowledge at the beginning of the fourth century BCE.