The investment legal regime is part of a broad landscape that encompasses various institutional arrangements and branches of the law. In some parts of the world, for example, supranational organizations have been created. The European Union is a prominent illustration. May member states of the European Union conclude investment treaties among themselves? International law, moreover, includes branches dealing with issues that have implications for investment law, such as human rights law. The chapter explores the ways in which investment law should cohere with the rest of international law, both vertically and horizontally, and how arbitrators should see their role in this fragmented legal environment.