When national courts judge international crimes like genocide, crimes against humanity and war crimes, they can draw on both national and international criminal law. The relationship between these two bodies of law is not always clear. Can national courts base prosecutions of international crimes directly on international criminal law? In a world where national laws often proscribe international crimes in an incomplete or deficient manner, this question has considerable practical relevance for the enforcement of international criminal law. Yet, it has received little attention in doctrine while practice shows widely divergent approaches of national courts to the feasibility of direct application. The author examines the concept of the direct application of international criminal law in national courts. He provides a rich description of the relevant practice in many different States ranging from Argentina to Senegal. Easily accessible, this book is a valuable tool for academics and practitioners alike.