This rich collection focuses on the broad research interests of Professor Nico Schrijver, in whose honour it was created. Written by a wide range of international scholars affiliated with Leiden University's Grotius Centre for International Legal Studies, the essays reflect Professor Schrijver's important contribution to academia and practice, particularly in the fields of sovereignty, human rights and sustainable development. The authors aim to reflect on changes in international law and on new developments in the diverse fields they explore. "Furthering frontiers" is the research theme of the Grotius Centre. Its exploration in this thought-provoking volume is a fitting homage to Nico Schrijver's achievements on the occasion of his retirement as Chair of Public International Law of Leiden University.