The strengths of international investment law – above all, a strong focus on investor interests and an effective adjudication and enforcement system – also entail its weaknesses: it runs the danger of impeding or even sanctioning the host states' legitimate regulatory interests and ignoring other fields of public international law. How does it cope with public interest concerns such as human rights, the environment or the fight against corruption? At the heart of this book lies a fresh approach towards a general theory of such global public interest considerations in the investment realm. Delineating how and why those considerations matter, and why the current system does not accommodate them properly, Andreas Kulick fleshes out general principles and customary international law as defences the host state may raise against alleged investor rights infringements and promotes proportionality as the appropriate balancing mechanism.