Although customary international law has long been an important source of rights and obligations in international relations, there has been extensive debate in recent years about whether this body of law is equipped to address complex modern problems such as climate change, international terrorism, and global financial instability. In addition, there is growing uncertainty about how, precisely, international and domestic courts should identify rules of customary international law. Custom's Future seeks to address this uncertainty by providing a better understanding of how customary international law has developed over time, the way in which it is applied in practice, and the challenges that it faces going forward. Reflecting an interdisciplinary mix of historical, empirical, economic, philosophical, and doctrinal analysis, and containing chapters by leading international law experts, it will be of use to lawyers, judges, and researchers alike.