This article explores the international humanitarian law governing the use of human shields, a tragically prevalent tactic in contemporary warfare. It begins by setting forth the express prohibitions on the use of human shields. These prohibitions are uncontroversial and generally accepted as reflective of customary international law. Much more problematic is the issue of how the use of human shields affects an attacker’s obligations under international humanitarian law. No treaty norm addresses the issue. This article distinguishes between compelled and voluntary shielding. It argues that those compelled to shield retain all the protections to which they are entitled as civilians during an attack. This result remains true despite the clear violation of international humanitarian law by the attacker’s enemy. By contrast, those who voluntarily shield a military objective qualify as “direct participants in hostilities.” As such, international humanitarian law removes their immunity from attack during the period of participation. Although there will seldom be a military reason to target voluntary shields, their status as direct participants does influence battlefield application of both the proportionality rule (prohibiting attacks expected to cause excessive collateral damage) and the legal requirement to take feasible precautions in attack (requiring attackers to take steps to minimize harm to civilians). This article concludes by suggesting that in cases of doubt as to whether shielding is voluntary, the shields should be treated as acting involuntarily.