This introductory Chapter sets the stage for the book by outlining the development and current state of international investment law as a rapidly evolving legal discipline. It highlights how the field has grown structurally and substantively through the proliferation of bilateral investment treaties (BITs) and the rise of investor-state dispute settlement (ISDS) mechanisms, which have played a critical role in shaping key protection principles such as Fair and Equitable Treatment (FET) and indirect expropriation. The Chapter frames the book’s central inquiry into how the interpretation of these substantive principles by arbitral tribunals interacts with states’ treaty-making practices, particularly in light of recent developments in new-generation international investment agreements like CETA and (CP)TPP. By establishing the framework, the Chapter underscores the ongoing challenges of legitimacy, coherence, and regulatory authority faced by international investment law, setting the foundation for a detailed examination of its legal-political evolution throughout the book.