Although the vast majority of drone attacks conducted by the United States have been signature strikes — strikes that target "groups of men who bear certain signatures, or defining characteristics associated with terrorist activity, but whose identities aren’t known"—scholars have paid almost no attention to their legality under international law. This article attempts to fill that lacuna. Section 2 explains why a signature strike must be justified under either international humanitarian law (IHL) or international human rights law (IHRL) even if the strike was a legitimate act of self-defence under Article 51 of the UN Charter. Section 3 explores the legality of signature strikes under IHL. It concludes that although some signature strikes clearly comply with the principle of distinction, others either violate that principle as a matter of law or require evidence concerning the target that the United States is unlikely to possess prior to the attack. Section 4 then provides a similar analysis for IHRL, concluding that most of the signature strikes permitted by IHL — though certainly not all — would violate IHRL’s insistence that individuals cannot be arbitrarily deprived of their right to life.