This book has a focus on the relationship between the principles of complementarity and universal jurisdiction. Territorial states are normally affected most strongly by core international crimes committed during a conflict or an attack directed against its civilian population. Most victims reside in such states. Most damaged or plundered property is there. Public order and security are violated most severely in the territorial states. It is also on their territory that most of the evidence of the alleged crimes can be found. There are, in other words, obvious policy and practical reasons why states should accord priority to territoriality as a basis of jurisdiction for core international crimes.