Today, most scholars put forward that one of the features of the ‘first’ war crimes trial at Nuremberg was a violation of the nullum crimen nulla poena sine lege principle. The idea took hold that, not only the crime of aggression but also, to a lesser extent, crimes against humanity were ex post facto law. This is not correct. A careful analysis of the IMT judgment and a renewed scrutiny of the sources of international law in existence at the time of the facts support a very different idea. Identical criticism is put forward against the modern ad‑hoc tribunals. A not so different picture unveils. The sources of international law and methods of interpretation used in Nuremberg continue to be legitimately used with similar results. Criticism is also largely unfounded.