Recent Euro 5 and Euro 6 vehicle emission standards are the first ever initiative to control particles on a number basis at the source.Related standards are also desirable for ambient nanoparticles (taken in this article to be those below 300nm) to protect against possible adverse effects on public health and the environment.However,there are a number of technical challenges that need to be tackled before developing a regulatory framework for atmospheric nanoparticles.Some of the challenges derive from a lack of standardisation of the key measurement parameters,including sampling,necessary for robust evaluation of particle number concentrations,especially in the context of insufficient knowledge of the physicochemical characteristics of emerging sources (i.e.bio-fuel derived and manufactured nanoparticles).Ideally,ambient concentrations of primary particles could be linked to primary particle emissions by use of nanoparticle dispersion models,and secondary nanoparticles using photochemical modeling tools.The limitations in these areas are discussed.Although there is inadequate information on the exact biological mechanism through which these particles cause harm,it is argued that this should not in itself delay the introduction of regulation.This article reviews the missing links between the existing knowledge of nanoparticle number concentrations and the advances required to tackle the technical challenges implied in developing regulations.