Customary international law remains the bedrock of international law. Its merits and demerits as compared with that other great source of the law, treaties, have often been discussed. It used to be thought in some quarters that custom would cease to be important as treaties multiplied. In recent years, a number of writers have expressed the view that custom had become irrelevant in modern international law But that was never a realistic view. As noted by Dinstein, ‘every once in a while, custom has a tendency to leap to the fore even within special fields of international law where it seemed to have ceded primacy to treaties long ago, thereby reminding the international legal profession that it is not just barely alive but is still pulsating with some intensity’. Even when whole areas of international law are codified in widely accepted international conventions, customary international law continues to play a vital role, as can be seen from the recent case-law of international and national courts and tribunals.

This book addresses the question of customary international law in the specific field of international investment law, and does so in a largely practical manner. It should be of great assistance to all those practising in this field, but it is equally of interest to those looking more generally at the formation and identification of rules of customary international law. As noted by d'Aspremont, ‘the scholarship on international investment law has remained bereft of theoretical reflection on the sources of investment law’. He says that ‘international investment law has now reached a stage of its development where the doctrine of sources can no longer be left in limbo and needs to be critically explored’ in order for this field of law to ‘rest on solid bases in terms of sources’. This is the aim of the present book.

One of the most notable features of international investment law is the overwhelming importance of treaties. Thus, more than 3,000 treaties containing provisions on investment protection have been signed by States. Treaties have become the dominant source of international law in this area. Why should one bother then with custom in this context?