This book has proposed a theorization of sustainable development in international law with the aim to make sense of the increased reference to sustainable development in international law more generally and in international investment agreements more specifically. It has found that sustainable development is a principled customary norm of international law, that steers behaviour of both States and corporations through integration of economic, social and environmental dimensions. In this way, sustainable development rebalances international investment law both horizontally and vertically, and frames corporate social responsibility.