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This article explores the two different kinds of uncertainty, ‘hard’ uncertainty (unknown unknowns) and ‘soft’ uncertainty (known unknowns), in the context of environmental law decision making. First, the authors argue that these different categories should not be treated the same when facing decisions under uncertainty. To deal with these different uncertainties, a tiered risk analysis process is called for, using participatory techniques to turn hard uncertainty into (more manageable) soft uncertainty as well as to increase the legitimacy of environmental decision making, even in cases of hard uncertainty. This methodology can and should apply to all instances of domestic, transnational and international environmental law making. This article applies this conceptual platform to analyze how participatory techniques can be factored in to manage uncertainty by reference to two domestic systems – American and Brazilian environmental law – as well as to international (environmental) law. The authors conclude that managing uncertainty in the environmental decision-making process is a procedural justice tool to promote more balanced and equitable outcomes.