A key concern in the resolution of investor-State disputes through arbitration is which law to apply to the substance of these disputes. It is not always straightforward if a claim brought by a foreign investor, a traditional subject of national law, against a host State, a traditional subject of international law, should be decided in accordance with national law, international law, or both. This essay reviews three books that consider how the lex causae in investment claims should be unraveled. Each book offers a different alternative to a conflict-of-laws approach in the determination of the governing law. Together, they reflect the diversity of views that animate the study and practice of international investment law.