International legal scholarship is concerned with the fragmentation of international law into specialised legal systems such as trade, environment and human rights. Fragmentation raises questions about the inter-systemic interaction between the various specialised systems of international law. This study conceptually focuses on the interaction between World Trade Organisation (WTO) law and external international law. It introduces a legal theory of WTO law, constrained openness, as a way to understand that interaction. The idea is that WTO law, from its own internal point of view, constructs its own law. The effect is that external international law is not incorporated into WTO law wholesale, but is (re)constructed as WTO law. It follows that legal systems do not directly communicate with each other. Therefore, to influence WTO law, an indirect strategic approach is required, which recognises the functional nature of the differentiated systems of the fragmented international legal system.