This chapter explores the footprint that temporary international legal regimes can leave on international law. Drawing on four different theories of state behaviour, it considers how temporary regimes can shape future permanent regimes. Under a rational design approach, temporary legal regimes influence future permanent regimes largely because they provide valuable experiences from which state actors learn. Under other theories of behaviour—historical institutionalism, constructivism, and behavioural international law—temporary legal regimes can have even more influence on permanent ones. Although these other three theories have important differences, all suggest that temporary regimes strongly shape the real and perceived possibilities for future permanent design choices. This chapter then looks at how these different theoretical approaches play out in case studies in refugee law, international criminal law, and international environmental law. While these case studies do not solely support any one theoretical account, collectively they demonstrate that temporary regimes can have outsized influence on permanent ones. This in turn has important implications for negotiators involved in regime design.