Numerous studies have found that elite and popular preferences influence decision making on the U.S. Supreme Court; yet, uncertainty remains about when, how, and why the Court is constrained by external pressure. I argue the justices are constrained, at least in part, because they fear nonimplementation of their decisions. I test this theory by utilizing a recent study of judicial power, which finds the Court enjoys greater implementation power in “vertical” cases (those involving criminal and civil liability) than in “lateral” cases (all others; e.g., those involving schools or government agencies). I find that Court constraint is strongest in important lateral cases—those cases in which implementation depends on support from nonjudicial actors. My findings suggest that Supreme Court constraint is driven by the justices' fear of nonimplementation and is, therefore, dependent on institutional context.