While in the beginning of the environmental debate, conflicts over environmental and technological issues had primarily been understood in terms of ‘‘risk’’, over the past two decades the relevance of ignorance, or nonknowledge, was emphasized. Referring to this shift of attention to nonknowledge the article presents two main findings: first, that in debates on what is not known and how to appraise it different and partly conflicting epistemic cultures of nonknowledge can be discerned and, second, that drawing attention to nonknowledge in technology conflicts results in significant institutional effects and new constellations of actors in public debates. To illustrate and substantiate this political dynamics of nonknowledge we draw upon examples from the areas of agri-biotechnology and mobile phoning. In a first step, we develop in greater detail the concept of scientific cultures of nonknowledge and identify three such cultures involved in the social conflicts within the two areas. Subsequently, we analyze the specific dynamics of the politicisation of nonknowledge looking at the variety of actors involved and the pluralisation of perceptions and evaluations of what is not known. Then, we point out some of the institutional reactions to the political and cultural dynamics of scientific nonknowledge. We argue that the equal recognition of the diverse cultures of nonknowledge is a key prerequisite for socially legitimate and ‘‘robust’’ decision-making under conditions of politicised scientific nonknowledge.