Paracelsus postulated that every substance is toxic and that only the dose makes the poison. The question is how to define a certain „dose“ for nanomaterials and for nanoparticles, respectively. Why is it impossible to calculate a distinct dose for nanoparticles? The problem is that nanoparticles are very diverse and heterogeneous regarding their chemical and physical properties. It seems rather unlikely that uniform units of measures or parameters characterizing these properties and reflecting the biological effectiveness could be developed. However, the dose calculation for nanoparticles is of high relevance, especially for risk assessment, limit values regulation and recommendations, respectively. Therefore this dossier outlines the correlation between exposition, dose and dose response and it explains why the knowledge of these crucial points is essential and where the gaps in knowledge still remain.