 Many critics allege that customary international law, with its reliance on vaguely-specified levels and combinations of state practice and opinio juris, renders the existence and content of international legal norms highly indeterminate, thereby violating the rule of law demand for notice and clarity (Goldsmith &amp; Posner 2005). Moreover, the process of reforming customary international law over time, which may involve states contravening existing customary norms in order to give rise to new norms, violates the rule of law requirement of congruence between law and official behaviour. Such criticisms have spurred efforts to construct accounts of the formation of customary international law that secure greater determinacy and that dispense with the need for violating existing custom in order to make new custom. But an alternative view is that greater fidelity to the rule of law requires that treaty law regimes should increasingly displace customary law (For contrasting views, see Bradley (ed.) 2016).