Foreign direct investment (FDI) screening regulations confer on public authorities broad powers and wide discretion to protect essential national interests against possible hostile investments driven by non-economic purposes. To this end, the power-conferring legal provisions include undetermined concepts concerning both the purpose and the scope of application. This analysis reveals how market operators are not only subjects of the screenings but also active players in the implementation of the regulations. Therefore, the operators must be able to understand and comply with the regulations. However, the use of general clauses and indeterminate concepts undermines the legal certainty of the regulations. Based on this consideration, this analysis first provides an overview of the principles of legal certainty and predictability in administrative law. Then, it considers the operators’ point of view and the example of the Italian FDI screening regulations to highlight the issues of legal certainty and predictability with regard to undetermined concepts. Finally, the analysis indicates how these issues risk undermining not only the attractiveness of the market for foreign investments but also the effectiveness of regulation, the compliance with the rule of law, and, eventually, the protection of public security.