Byzantine medicine is the biggest unknown quantity in both the study of medieval medicine and science and the wider field of Byzantine studies. This volume presents the first comprehensive examination of the medical corpus of, arguably, the most important Late Byzantine physician: John Zacharias Aktouarios (c.1275-c.1330). Its main thesis is that John’s medical works show an astonishing degree of openness to knowledge from outside Byzantium combined with a significant degree of originality, in particular, in the fields of uroscopy and human physiology. Analysing his theories not only enhances our understanding of the Late Byzantine social and intellectual landscape, but also sheds new light on the broader medieval Mediterranean context, highlighting the cultural exchange between Byzantium and its neighbours.