In an attempt to identify elements that all criminal prohibitions have in common, many theorists, most of whom have been deemed adherents of criminal law theory, have proposed a number of formal requirements for criminal liability. One of these requirements--most often called the requirement'--is that there must be, among other things, an act on the part of the offender in order for that offender to incur criminal liability. (1) Though there is considerable disagreement over what should count as an overt act, one strategy taken by many theorists is to define an act as a bodily In an attempt to resuscitate the act requirement in the face of criticism of the orthodox position, Michael S. Moore has reaffirmed a similar understanding of the act requirement that defines an act as a volition causing a bodily movement. (2) As a corollary to this understanding, Moore develops a pair of arguments--one metaphysical and the other normative--against liability for most omissions. In this essay, I argue that Moore's treatment of omissions is flawed and that its flaws stem from Moore's broader theory of action, particularly from his commitment to defining an act as a volition causing a bodily movement. In particular, I contend first that Moore's metaphysical claims about omissions are incompatible with his own normative argument against omissions liability, and second that in both his metaphysical and normative claims concerning omissions, Moore fails to recognize the significant and legitimate role that positive duties--requirements to perform specific actions--play within the criminal law. In the end, even though I agree that there is an act requirement in the criminal law, it is my position that the legitimacy of positive duties within the criminal law necessitates an understanding of the act requirement that is incompatible with volition-based accounts like the one Moore defends. I Moore's Two Claims Against Omissions One line of criticism used by theorists against the orthodox account of an act is that it implies that the act requirement necessitates actions--actual bodily movements--for criminal liability. If this is true, then the orthodox account would seem to preclude, among other things, the possibility that any kind of omission could be held criminally liable. Precluding liability for omissions is problematic primarily because there are many criminal proscriptions of omissions to be found in the present practice of Anglo-American law, very few of which are controversial. Of course, present practice may be theoretically inconsistent or even internally contradictory, but there seem to be good reasons for many of the criminal prohibitions of certain omissions. Few would deny, for example, that the protection of children is a legitimate and important reason behind criminal statutes proscribing the failure of parents to take adequate care of their children. Similarly, statutes prohibiting tax evasion help to guarantee the maintenance of infrastructure and the provision of (at least minimal) social services. As such, a theorist arguing against such prohibitions must demonstrate that laws proscribing omissions either appeal to some principle that deeply contradicts other established principles that underlie the criminal law as a whole or appeal to no principle at all, and thus should be rejected. Moore attempts to demonstrate just such a contradiction (even though he does concede that criminal prohibitions against failing to take adequate care of one's children constitute justifiable exceptions to his view). (3) Moore argues against treating omissions in the same ways we treat positive actions within the criminal law. His argument stems from the orthodox approach to the act requirement, requiring a willed bodily movement, but strays from orthodoxy in that he is unwilling to acknowledge the importance of the proscription of some omissions in the criminal law that some of his predecessors in orthodoxy (Austin, Bentham, and Salmond) accept quite readily. …