During the late Middle Ages and into the Renaissance, fighting books—Fechtbücher—were produced in northern Italy, among the German states, in Burgundy, and on the Iberian peninsula. Long dismissed by fencing historians as “rough and untutored,” and largely unknown to military historians, these enigmatic treatises offer important insights into the cultural realities for all three orders in medieval society: those who fought, those who prayed, and those who labored. The intent of this dissertation is to demonstrate, contrary to the view of fencing historians, that the medieval works were systematic and logical approaches to personal defense rooted in optimizing available technology and regulating the appropriate use of the skills and technology through the lens of chivalric conduct. I argue further that these approaches were principle-based, that they built on Aristotelian conceptions of arte, and that by both contemporary and modern usage, they were martial arts. Finally, I argue that the existence of these martial arts lends important insights into the world-view across the spectrum of Medieval and early Renaissance society, but particularly with the tactical understanding held by professional combatants, the knights and men-at-arms. Three treatises are analyzed in detail. These include the anonymous RA I.33 Latin manuscript in the Royal Armouries at Leeds; the early German treatise attributed to Hanko Döbringer that glosses the great Johannes Liechtenauer; and the collection of surviving treatises by the Friulian master, Fiore dei Liberi. Each is compared in order to highlight common elements of usage that form the principles of the combat arts.