Public international law forms an umbrella framework under which its various substantive areas of law sit. The fragmentation this represents is a well-known phenomenon in international law and its implications are manifold.1 A key aspect of this fragmentation and increasing specialisation is the potential for competing norms to encounter each other within the international legal space. There seem to be several mechanisms for resolution of this available in theory, and not a great deal of consistency in practice. One area of contestation in particular has attracted controversy, largely due to the public interest issues implicated, the asymmetries in dispute settlement approaches and the prioritising of one set of norms over another – environmental law, policy and protection objectives and international investment law. The environment/investment nexus became a high-profile international issue through a number of coinciding, parallel channels. On one level, the damaging effects of the activities of multinational corporations on the environment and the health and well-being of local communities formed the backdrop against which international legal issues would be played out. Catastrophic examples of this mode of encounter included the Bhopal disaster, the ‘dieback’ experienced downstream from the BHP copper and gold mine at Ok Tedi and Chevron/Texaco’s leaching of crude oil into the Amazonian ecosystem. At the same time as such micro-level incidents were occurring, global environmental issues that involved multinational corporate operations, such as climate change and the need for the widespread adoption of policies aimed at achieving sustainable development, were appearing in international instruments. The environment/investment nexus also became particularly visible in the late 1990s in the context of investor-state arbitration, when environment- related investment disputes began to be filed with international tribunals, which then triggered extensive protests regarding the negotiation of a Multilateral Agreement on Investment under the auspices of the Organisation for Economic Co-operation and Development (OECD). From that point onwards, the interaction between the treatment of environmental issues and norms and the rules contained within international investment agreements remained controversial.