Why devote a voluminous book to the issue of mistake of law in international criminal law? Would it not appear frivolous for a person accused, for example, of crimes against humanity to claim that he did not know that what he did was against the law? And even if he was in fact mistaken, why should the law care as long as the perpetrator was aware of what he was doing? Annemieke van Verseveld brilliantly lays to rest such skeptical queries about the need for her study. In this book, she shows that it is not only worthwhile but even urgently necessary to reflect on mistakes of law. She demonstrates the relevance of her inquiry by showing, in Chap. 6, a host of areas of international criminal law where the applicable law is far from clear. And, based on a comprehensive analysis of four national legal systems (USA, England, Germany, and France) as well as of international criminal law, the author shows that the law in regard to mistakes of law is presently in disarray and lacks a clear direction. Annemieke van Verseveld’s concise but thorough treatment of the topic, therefore, is timely and useful. Any practitioner who deals with international criminal law cases should read this book. But that is not all. The issue of how the law should react to a defendant’s claim that he was not aware of the norm he violated raises fundamental questions about the purpose and structure of the criminal law. Does the intent required for conviction of a crime include knowledge of the relevant criminal prohibition, or of the moral wrongfulness of one’s conduct? Should the perpetrator’s mistake of law affect his culpability rather than the required mens rea? Or can such mistakes simply be ignored, in accordance with the maxim ignorantia iuris neminem excusat?