The Supreme Court's Fourth Amendment doctrine regulating police violence, including its recent decision in Scott v. Harris, is unprincipled and indeterminate. The common law of justification defenses, by contrast, provides a well-established legal structure for determining when one person may justly use force against another. In this Article, I argue that this structure should be imported and adapted to the constitutional doctrine governing police uses of force. Following the structure of justification defenses, I contend first that police uses of force can be constitutionally justified only if they are in pursuit of legitimate state interests. In particular, police uses of force are justified only if they assist the mechanisms of criminal justice (e.g., arrests), preserve public order, or protect officers from harm. Continuing the analogy to justification doctrine, I also maintain that even when used to pursue one of these legitimate ends, police uses of force are constitutionally justified only if they (1) respond to an imminent threat to one of these ends; (2) reasonably appear to be necessary in degree and kind to defend against that threat; and (3) create a risk of harm that is not substantially disproportionate to the interest they serve. These concepts of imminence, necessity, and proportionality cannot be imported wholesale from justification law, however. Instead, they must be tailored to accommodate important differences between the police and ordinary civilian defenders and refined over time by courts in the context of cases alleging excessive force. Suitably modified, the justification framework can operate within the Fourth Amendment jurisprudence to provide a more reasoned, predictable, and just assessment of police violence, one that takes police officers seriously both as state actors and as vulnerable and limited human beings.