The exchange, adaptation and diffusion of ideas has long been part of planners' modus operandi. One of the most interesting and widespread of the new pro- cedures of the last quarter century has been that of 'environmental impact assessment* (El A). In its concern with tracking, measuring and evaluating, it shares many methodological characteristics with other forms of impact assessment (for example, social impact), and to that extent is part of a broader field. In its immediate role and content, however, El A also reflects the different local institutional contexts in which it has to operate and the growing concern with the natural and built environment manifested as global concerns in the UN summits at Stockholm, Vancouver and Rio. It has also brought together a new and shared set of interests and expertises between natural scient- ists, engineers and planner