John Caius (1510-75) enjoyed a European reputation as a Galenist physician. This study, based on his marginalia preserved in Eton and Cambridge, describes Caius' immense efforts to see and collate medical manuscripts in Italy and England over almost two decades. His reports are important for a modern editor of Galen, since many of these 'codices' are, apparently, now lost, and some were of high quality. Caius' notes also shed light on the growth of medical humanism, on the accessibility of Greek books and manuscripts in the fifteenth and sixteenth centuries, and on the methods of Renaissance editors of Greek technical prose texts. Caius' evidence also prompts a reassessment of the 1525 Aldine Galen, and of the activities of two of its editors, John Clement and Edward Wotton. This study is of importance to students of both ancient medicine and the transmission of Greek learning in the West.