The report describes the considerations, essential steps and organisational aspects of the development of the ⁠OECD⁠ Test Guideline for the determination of particle size and particle size distributions of nanomaterials. It gives furthermore insights into the selection, preparation and pre-validation of test materials used in an interlaboratory comparison and presents the results of this international laboratory comparison tests and their impact on the final version of the OECD Test Guideline. Upon adoption at OECD level, the Test Guideline for the determination of particle size and particle size distributions of nanomaterials will be available at the webpages of the OECD Test Guideline Programme.