This study argues that treason is understood as a breach of allegiance in medieval popular tradition as well as in legal definitions of the crime in Roman, Anglo-Saxon, military, and medieval French and English law. The scope of treason in Malory's Morte Darthur owes much to the crimes of treason in military, English, and archaic French law. But Malory also reflects extra-legal acts of treason such as adultery. He synthesises from these diverse laws and ideas a reasonably consistent body of pseudo-historical custom, which contributes to his Arthurian society's material plausibility and realism. Malory's treatment of the traitor is greatly indebted to extralegal thought, most notably in that his traitors are evaluated in terms of their motivations and ethical characters as well as their culpability of objective traitorous acts. Malice, mortal sin, unnatural tendencies and repeated treasons characterise the traitor as villain: the traitor as hero is depicted as fundamentally virtuous, non-malicious, and generally commits one treason only with the best of motivations. Treason, however, always involves sin, and in the last three tales Malory begins to acknowledge that treason therefore implies a crime against God as well as society. Infidelity to God in the last two tales is expressed through the coinciding treasons, disloyalties and overvalued worldly loyalties of Malory's characters, and these, regardless of the moral intentions of the perpetrators, bring about the downfall of the Arthurian kingdom. The fall of the nation can be interpreted as a retribution for the characters' sins against God which leads the surviving members to realign their allegiances and embrace heavenly chivalry and the religious life in recognition of and in penance for their previous misdeeds.