This article describes unresolved challenges of regulation of nanoparticles, mainly from a European perspective. The authors shall argue that the presently dominant strategy, which in this article will be called 'filling in the regulatory gaps', is unsatisfactory with respect to the management of uncertainty and ignorance associated with development and use of nanoparticles. Its shortcomings resemble those of 'the modern model' and 'the precautionary model' described by Funtowicz (2006) and

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... owicz and Strand (2007) . Possible future alternative strategies to enable a truly precautionary approach are discussed.