The practice of carrying out an environmental impact assessment (EIA) has gained strength in international law and jurisprudence, finding application in the case law of the International Court of Justice (ICJ). If, on the one hand, the ICJ has recognized the customary nature of this principle, on the other its application poses a set of challenges, mainly linked to the autonomy of this obligation from other international environmental law norms. More precisely, the obligation at issue has been applied in connection with the due diligence and notification principles, creating uncertainty about its scope, as well as about its substantive or procedural nature. Likewise, the autonomy of the obligation to perform an EIA has been challenged in relation to the definition of the content and scope of the obligation itself, which in turn is linked to the existence of applicable treaty provisions or of soft law. This article discusses the impact of these elements on the reasoning of the Court in the cases at issue, in order to demonstrate how such lack of autonomy can undermine the coherence of the reasoning itself and, therefore, an effective application of the principle.