This Essay offers a unified framework for understanding how law can protect a vulnerable person from a powerful one. One option law has is to penalize the powerful person if she harms the vulnerable person. This option can be called a “harm rule.” But sometimes law shifts its focus from regulating the infliction of harm to regulating a person’s accumulation of power to inflict harm. Legal rules that reflect this shift in focus can be called “power rules”; they expressly restructure underlying relations of power and vulnerability. Power rules allow legal regulation of situations in which rules directly regulating harm are not possible. Even when harm rules are possible, power rules can complement harm rules and improve their effectiveness. But power rules have drawbacks, too: They tend toward overbreadth, encourage merely expressive lawmaking, and increase enforcement discretion. The concept of power rules helps explain patterns in the use of legal rules, especially in the contexts of bargaining, competition, violence, persuasion, and the performance of relational statuses (e.g., status as a fiduciary). This concept also illuminates the tradeoffs involved when lawmakers choose among different methods of protecting vulnerable persons.