In December 2021, the German Environment Agency (UBA) published a report entitled Development of a specific OECD Test Guideline on Particle Size and Particle Size Distribution of Nanomaterials. The UBA commissioned the Federal Institute for Occupational Safety and Health (BAuA) (project coordinator) and the Federal Institute for Materials Research and Testing (BAM) to develop an Organization for Economic Cooperation and Development (OECD) Test Guideline for the determination of particle size and particle size distributions of nanomaterials. The report describes the considerations, essential steps, and organizational aspects of the development of the Test Guideline. It provides insights into the selection, preparation, and pre-validation of test materials used in an interlaboratory comparison and presents the results of the 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 on the website of the OECD Test Guideline Program. The draft Test Guideline is available online.