International investment law has rapidly evolved from a relatively recent field grounded in longstanding legal and political traditions. The rise of investor-state dispute settlement (ISDS) mechanisms has significantly shaped the interpretation and development of key protection principles, creating a feedback loop between arbitration and treaty-making. This book examined how new-generation international investment agreements (IIAs) reflect lessons learned from arbitral practice, especially regarding indirect expropriation and Fair and Equitable Treatment (FET). Through detailed case studies, it highlighted the ongoing balancing act between state sovereignty, investor protections, and public interests. Ultimately, the book underscored the dynamic process of legal evolution and the challenges ahead in achieving clarity and fairness in international investment law.