This chapter discusses the question specific to the establishment of international boundaries. After defining boundaries, the author notes the fundamental importance of the consent of the disputing parties to the establishment of the boundaries, dismissing any automatic reliance upon the theory of ‘natural boundaries’. The chapter also examines hypotheses when the boundary is established by application of the principle of uti possidetis. Giuseppe Nesi also analyses the role of third parties in the determination of a boundary, such as judges and international organizations. Finally, the author reviews the rules protecting boundaries in international relations.