The introduction presents the subject matter of the book, the structure of the volume, and the questions it attempts to answer. Among these questions, the book investigates the relation and extent of complementarity between VSS and international law rules in the social and environmental domains, as well as trade. It aims to understand the modalities of public use of VSS in market regulation, their complementarity with EU rules and their implications. The book then studies how EU internal market and WTO legal regimes can control, coordinate and review VSS. In particular it proposes a possible interpretation of EU and WTO legal rules and meta-rules, in view of addressing the trade barrier effects of VSS and to possibly scale-up their effectiveness.