International law is in constant expansion and seeks to regulate matters in a variety of fields, such as human rights, investment, trade, international criminality and regional economic integration. The multiplicity of these regimes and their elaboration in isolation from one another has lead to a state of “fragmentation” of international law. In that regard, examples of self-contained regimes are numerous. Nonetheless, certain similarities in the ways in which international law is designed and construed by international courts allow judges and academics to look for common features, since they are often confronted with the same problems and use similar methods to deal with them. It is in the context of the COST action IS1003 International Law between Constitutionalization and Fragmentation: The Role of Law in the Post-National Constellation that Lukasz Gruszcynski and Wouter Werner decided to realize a collective work on these similarities with regard to judicial deference.