This article discusses whether the Rules formulated in the Customary International Humanitarian Law Study are automatically a part of national law, particularly in common law States. It argues that even if some or all of them accurately reflect customary international law at the present time, their status is only as potential norms of national law. It concludes that they cannot, by themselves and without more, amount to crimes within national law. The author discusses also the relationship between the Rules at common law and any wholly or partially implemented treaty upon which they are based. He concludes that the Rules, as such, will generally have little practical impact upon national criminal law.