This volume asks a question that is deceptive in its simplicity: Could international law have been otherwise? In other words, what were the past possibilities, if any, for a different law? The search for contingency in international law is often motivated, including in the present volume, by the refusal to accept the present state of affairs and by the hope that recovering possibilities of the past will facilitate a different future. The volume situates the search for contingency theoretically and within many fields of international law, such as human rights and armed conflict, migrants and refugees, the sea and natural resources, and foreign investment and trade. Today there is hardly a serious account that would consider the path of international law to be necessary and that would deny the possibility of a different law altogether. At the same time, however, behind every possibility of the past stands a reason – or reasons – why the law developed as it did. Those who embark in search of contingency soon encounter tensions when they want to recover past possibilities without downplaying patterns of determination and domination. Nevertheless, while warring critical sensibilities may point in different directions, only a keen sense of why things turned out the way they did makes it possible to argue about how they could plausibly have turned out differently.