This guidance is directed to registrant(s) who have identified several nanoforms of the same substance3 and need either to determine to which forms (including non-nanoforms) the available hazard data will be applicable and/or to design a testing strategy to fulfil the information requirements for the substance. Additionally, the principles laid out in this document will also be useful for a potential registrant joining an existing registration that includes (nano)forms and who needs to determine whether the hazard data available could also be applicable for the nanomaterials he is putting on the market.