National constitutions exhibit a wide variety of approaches to international law. Some constitutions provide that customary international law is directly incorporated into the constitutional order, while others do not even mention customary international law. Some constitutions establish elaborate treaty-making processes, while others have relatively simple processes involving one or two actors. We have to date very little theory as to why countries differ on these dimensions. This article draws on the literature on constitutions as pre-commitment devices, and examines the unique features of international law that facilitate policy entrenchment. It argues that constitution-writers design the interface with international law to facilitate an optimal level of commitment. In particular, international law is useful to constitution-writers in new democracies, who have relatively limited domestic mechanisms to commit themselves to policies. The article draws on a new database of national constitutions to provide empirical evidence for these propositions.