This article explores the legal principles that govern the interpretation of “secondary instruments” in international law. A “secondary instrument” under international law is, for the purposes of this article, a written document adopted by a body empowered by a treaty to take action with respect to the treaty, but which is not itself a treaty. Such instruments find increasing application in international law. The article specifically examines the interpretation of secondary instruments arising in five settings in international practice: the United Nations Security Council, the International Maritime Organization, the International Seabed Authority, the International Whaling Commission, and conferences/meetings of the parties under multilateral treaties. This selection of practice will serve to illustrate principles of interpretation across a range of international institutional settings for the purpose of determining the rights and obligations of state-parties to a treaty regime.

The present contribution pays tribute to the late Professor Janusz Symonides by examining the position of United Nations Security Council towards international terrorism. The analysis concentrates on how the phenomenon is perceived by the main political organ of the United Nations, and offers some cursory remarks on its reactions (both actual and potential).