Foreign investments are usually implemented through contracts between host States and foreign investors. These contracts and international investment treaties represent two different legal instruments that protect foreign direct investment. The co-existence of both instruments under international investment law has generated fundamental problems. By scrutinizing and tracing the increasingly divided jurisprudence on central aspects of treaty interpretation and analyzing the conflicting legal concepts applied by arbitral tribunals, this book represents a comprehensive examination of the complex relationship between the two in the field of investment treaty arbitration.