Customary international law is the most important source of international criminal law. Fifty years after the Nuremberg trials, many convictions imposed by the tribunals for the former Yugoslavia and Rwanda are still based on customary international law alone. The International Criminal Court, by contrast, has not yet had much opportunity to give more guidance on this matter. Hence, it is worthwhile to provide an overview of the current status of custom by analysing the ad hoc tribunal's case law on this point. Including a comprehensive synopsis of current literature and a contrast of the ad hoc tribunal's case law with the jurisprudence of the International Court of Justice, this book offers an inclusive insight into the source's past and future.