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Since the middle of the twentieth century, the field of international investment protection has gone through a period of more or less continuous expansion. From a single bilateral investment treaty (‘BIT’) signed between Germany and Pakistan in November 1959, international investment law has seen the proliferation of some 3,200 investment treaties governing the treatment of foreign investors by the host States where they do business.

As a historical matter, the substantive elements of modern investment law emerged from a loose network of customary international law protections that pre-existed the treaties now dominating the regime. Customary international law had long required host States to extend certain guarantees of decent treatment to foreign citizens within their jurisdiction. The systematic codification of these customary norms into a far-flung network of treaties began in earnest with the late nineteenth century emergence of so-called ‘friendship, commerce, and navigation’ treaties, which incorporated existing customary rules and adopted various new substantive requirements. The treaty network took its next step when BITs proper emerged in the mid-twentieth century, characterised principally by the extension of dispute resolution options to individual investors.

As customary investment law was gradually codified at the retail level, the law of treaties began to loom much larger in meta-regulation of the regime. This chapter will explore some of the ways that the modern law of treaties interacts with the modern law of international investment protection. It will focus in particular on a handful of areas where the formal categories of treaty law map awkwardly onto the reality of modern investment law and adjudication.


This material has been published in Conceptual and Contextual Perspectives on the Modern Law of Treaties, edited by Michael J. Bowman and Dino Kritsiotis, DOI: . This version is free to view and download for private research and study only. Not for re-distribution or re-use. © Cambridge University Press.