In May 2025, the Core Group of states finalized the technical work for treaties to establish the Special Tribunal for the Crime of Aggression against Ukraine. This tribunal, to be created through a treaty between the Council of Europe and Ukraine, will derive its jurisdiction from Ukraine. The choice of the Council of Europe as a partner in creating the tribunal has significant legal consequences in international treaty law as well as in international criminal law and beyond. In performing its functions and upholding international justice, the tribunal will confront complex legal challenges. This book explores those challenges in depth.