This article examines evaluates the strength of the Labor-Environmentalist alliance of the late twentieth century. It traces the evolution of trade unionists' thinking about nature and the human relationship to the environment by examining intellectual and political sources of labor involvement in United Nations' environmental policy making from the 1950s through the 1980s. The article explores the reasons trade union organizations, notably the International Confederation of Free Trade Unions, the International Trade Secretariats (Global Union Federations) and the European Trade Union Confederation, participated in a variety of international conferences and institutions such as the 1972 Stockholm Conference on the Environment, the 1992 Rio Earth Summit and the 2002 Johannesburg World Summit on Sustainable Development. It finds that environmentally conscious trade unionists developed their own version of environmentalism and sustainable development based on a reworking of basic trade union principles, a reworking that emphasized solidarity with nature and made central the protection of the health and safety of workers, communities, and environments.