This volume explores the versatility of the concept of pneuma in philosophical and medical theories in the wake of Aristotle’s physics. It offers fourteen separate studies of how the concept of pneuma was used in a range of physical, physiological, psychological, cosmological and ethical inquiries. The focus is on individual thinkers or traditions and the specific questions they sought to address, including early Peripatetic sources, the Stoics, the major Hellenistic medical traditions, Galen, as well as Proclus in Late Antiquity and John Zacharias Aktouarios in the early 14th century. Building on new scholarly approaches and on recent advancements in our understanding of Graeco-Roman philosophy and medicine, the volume prompts a profound reevaluation of this fluid and adaptable, but crucially important, substance, in antiquity and beyond.