An environmental impact assessment (eia) can have a valuable role in effectuating substantive rules of international law of the environment, such as the obligation of due diligence and the precautionary approach. Thus this article seeks to examine the interlink between an obligation to perform an eia, the obligation of due diligence, and the precautionary approach, focusing on temporal elements of international law of the environment. First, this article examines the interlink between an obligation to conduct an eia and the obligation of due diligence. Second, this article addresses the interlink between an obligation to conduct an eia and the precautionary approach. Third, the timing of triggering the obligation to conduct an eia must be considered. Finally, this article examines the issues of breach of the obligation to conduct an eia, before offering conclusions.