The structural role of law-of-war perfidy is widely unappreciated and misunderstood. More than a prohibition of underhanded or dishonorable conduct, the prohibition of perfidy is an essential buttress to the law of war as a medium of exchange between combatants – a guarantee of minimum respect and trust between belligerents even in the turmoil of war. Indeed, it may be difficult to conceive of an operative or effective war convention at all without guarantees against and protections from perfidy. Yet most existing conceptions of perfidy, whether drawn from treaty, military legal doctrine, or legal scholarship, merely restate imprecise codifications or offer little more than a vague sensibility. Amid seismic shifts in the conduct, scale, participants, and means of warfare, States have codified progressively narrower conceptions of perfidy, ultimately incorporating discrete legal elements into the offense. This article argues that a principled conception of perfidy that protects minimal concerns of humanity and preserves the law of war as a scheme of minimum good faith between adversaries is at once highly elusive but critical to the future of regulated violence.