Environmental Impact Assessment (EIA) is a legal device used by many States to protect the environment. Such assessments can bring foreign investors into conflict with host State regulators and civil society. When this occurs, an investor may seek to protect its investment against the consequences of EIA by pursuing remedies under an international investment agreement (IIA). The purpose of this article is to examine the role of IIAs in maintaining and supporting the integrity of the EIA process of the host State. It focuses on ways to integrate EIA into IIAs in order to strengthen the State’s ability to protect the environment. The article examines recent case law to identify ways in which EIA can give rise to investor-State claims under IIAs. It also discusses two theoretical models that are useful for designing reforms: the global administrative law paradigm and the cooperative transnational approach.