In spite of early hopes for a “fading out” of sovereignty in the face of global environmental challenges, recent codifications of international law have confirmed the creeping national enclosure of what were once considered common assets—e.g., exclusive economic zones under the 1982 Law of the Sea Convention, or access to genetic resources from the 1983 International Undertaking via the 1992 Biodiversity Convention to the 2001 Plant Gene Treaty. Yet, because of their explicit limitation and qualification by “common interest” obligations, these expanded sovereign rights of nation states must be considered fiduciary rather than proprietary. Hence, the emerging legal regime is one of international public trusteeship (sometimes referred to as guardianship or stewardship) over a widening range of environmental resources. The article traces the evolution of the trusteeship concept in modern environmental law and its ramifications for international law and governance, as reflected in current proposals suggesting a new environmental mandate for the UN Trusteeship Council.