Courts and commentators commonly claim that criminal law contains a voluntary act requirement. Despite the ubiquity of this assertion, there is remarkably little agreement on what the voluntary act requirement entails. This lack of uniformity is particularly problematic because, for some crimes, whether a defendant is guilty or innocent will turn on which conception of voluntariness is applied. In this Article, we critique the various conceptions of the voluntary act requirement, and propose an alternative set of principles for applying the notion that person is only criminally culpable for crimes committed voluntarily. First, culpability requires that the actus reus as a whole (rather than merely one element of the actus reus) be voluntary. Second, the voluntariness requirement is an affirmative element of every offense, with the prosecution bearing the burden of proving voluntariness. Third, the Constitution requires that voluntariness is a necessary condition of criminal liability. These principles resolve the inconsistent understandings of the voluntariness requirement and ensure that criminal liability is limited to those defendants who are responsible for prohibited activity.