Recent research shows that interpretations and uses of the precautionary principle can exacerbate the relations between states. Indeed, the precautionary principle increasingly plays a pivotal role in explaining how actors position themselves on various issues related to health, safety and environmental (HSE) risks. However, the books under review tend to subscribe to the approach that separates out precaution from risk assessment. In contrast, it is suggested in this article that the precautionary reasoning is a distinctive view of risk regulation, which reworks rather than repudiates risk assessment. This turn might help scholars to embark on the difficult task of understanding the extent to which the precautionary principle affects how HSE norms on risks are crafted, the channels through which they spread and the mechanisms that enable them to operate effectively.In the last decade, a series of controversies on health, safety and environmental risks (HSE) has pitted two of the giant economic actors in the international system, the European Union (EU) and the United States (US), against one another. The disagreement is over the criteria that should underlie risk regulation at the international level. Although both sides agree that a broad family of risks is characterised by radical uncertainty and ignorance about the depth and magnitude of future consequences, they spar over the norm that policy makers should trust in order to fashion policies that forestall disastrous outcomes.In general, it is argued that the EU places its faith on the precautionary principle while the US insists on cost-benefit analysis. Put in simple terms, risk assessment aims to turn uncertainty into knowledge. Precaution, by contrast, considers that not all nonknowledge can go through this transformative process. Yet, precaution has been conceptualised in many different ways. One of the best known, if not most influential formulations of the precautionary principle is contained in Principle 15 of the 1992 Rio Declaration: ' [W]here there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation.'The stakes are high. Indeed, from trade to war, through the discussion on the appropriate steps to take in order to minimise the effects of global warning, the tensions between cost-benefit analysis and the precautionary principle are changing the ways in