When a perpetrator of an international crime argues in his defence that he did not realise that he had violated the law, is this a reason not to punish him? International crimes constitute serious offences and it could be argued that he who commits such an offence must know his act is punishable. After all, everyone is presumed to know the law. However, convicting someone who is mistaken about the wrongfulness of his act may be in violation of the principle ‘no punishment without guilt’. This book investigates when 'mistake of law' should be a reason to exculpate the perpetrator of an international crime. It demonstrates that the issue of 'mistake of law' goes to the heart of individual criminal responsibility and therewith contributes to the development of a more systematic approach toward the structure of international offences. Valuable for academics and practitioners in the field of International Criminal Law.