The main purpose of this document is to assist the regulated community and regulators in selecting the most appropriate acute inhalation TG so that particular data requirements can be met while reducing animal usage and suffering. This Guidance Document also contains additional supporting information on the conduct and interpretation of studies performed using the inhalation test guidelines: TG 403, TG 436, TG 433, TG 412, and TG 413. As mentioned before, for a number of Test Guidelines, such as TG 451 (carcinogenicity), TG 452 (chronic toxicity) or the Test Guidelines on reproduction and neurologic endpoints, the inhalation route of exposure may be used but this is not mentioned specifically in the guidance. If the inhalation route is chosen, the present guidance document should be consulted in the design of the studies. The second purpose of this document is to provide the necessary guidance when testing nanomaterials via inhalation in the 28-d and 90-d toxicity studies, subsequent to the high priority activity undertaken at the OECD.