Through the precautionary principle, governments acknowledge the limits of science as a basis for policy, while seeking to clarify scientific uncertainty. This tension is exemplified by the European risk regulation of genetically modified (GM) crops. The risk debate has been translated into various precautionary approaches, each with its own cognitive framing of the relevant uncertainties. Early safety claims took for granted intensive agricultural models; normative judgements served to downplay uncertainties which were not readily reducible, thus justifying commercial approval of products. In the late 1990s public protest strengthened broader accounts of uncertainty, for example through more stringent environmental norms and more complex causal pathways of potential harm. Fact-finding methods were debated as a value-laden choice for how best to generate more relevant knowledge.As risk-assessment research challenged assumptions in safety claims, critics cited the results as evidence of greater uncertainty. Invoking the precautionary principle, regulatory procedures delayed or restricted commercial use of GM crops. They not only increased the burden of evidence for safety, but also stimulated and requested knowledge about more complex uncertainties. Criteria for relevant evidence were implicitly linked with different framing visions for agriculture.Such value conflicts made scientific uncertainty more important - rather than vice versa. When risk research methods were challenged, fact/value boundaries were blurred, thus increasing 'uncertainty' - rather than vice versa. In these ways, the risk controversy was constituted by divergent accounts of the relevant scientific uncertainty. Uncertainty was constitutive, not merely contextual. In general, then, precaution offers a means to justify uncertainty - not simply vice versa.