In recent decades, environmental sciences and technologies have greatly expanded our understanding of ecosystems and their functioning. International environmental treaty regimes have progressively incorporated this emerging body of knowledge through various ‘boundary objects’, concepts sitting at the interface of science and the law. The most important of these concepts is arguably the ecosystem approach. Compared with the significant expectations it generates, however, the approach is still underexplored in terms of its actual influence on the evolution of international environmental law. To fill this gap, this dissertation studies the legal expression of the ecosystem approach in a representative set of multilateral environmental agreements (MEAs) and explores how the science underpinning the concept has been translated into legal norms. The dissertation adopts an interdisciplinary method which combines legal analysis with a broader historical account of the co-evolution of science and international environmental law. Overall, the dissertation shows that in implementing the approach, MEAs have privileged some types of legal expression over others. In particular, there has so far been an emphasis on the creation of procedural rules and obligations of conduct over the setting of positive obligations to maintain ecosystem integrity. The dissertation argues that this failure to translate ecosystem integrity considerations into legal norms is linked to structural limitations imposed by some of the fundamental building blocks of international environmental law, and especially by the sovereign right of States to exploit biological resources located under their jurisdiction.