This document reviews characterization of CNT and CNFaerosols for inhalation exposure studies. The document also provides useful information on appropriate characterization of CNT and CNF, which is required to evaluate and understand the inhalation toxicity of CNT and CNFaerosols. This document neither provides guidance on aerosol characterization for other carbon nanomaterials, nor provides guidance for characterization of carbon nanotube and nanofibreaerosols in the workplace or ambient air.