The Costa Rica - Nicaragua cases, which led to the judgment of the International Court of Justice of 2 February 2018, are rich in lessons. The International Court, in its judgment of 16 December 2015, specifies the content of the international obligations of "due diligence" regarding the prevention of environmental damage. Step by step, the Court continues to consolidate an international environmental law of customary nature around the “due diligence” principle, which forms its bedrock. Its decision of 2018 gives it the opportunity to recognize environmental damage in international law and to admit its compensable character. Despite some grey areas, these cases brought a substantial contribution to the development of international environmental law.