In December 2017, Germany’s Federal Environment Agency (UBA) published a report entitled Clarification of methodical questions regarding the investigation of nanomaterials in the environment: Development of a decision support tool for the investigation of nanomaterial’s environmental behaviour based on dispersion behaviour and dissolution in relation to various environmental parameters. UBA states that for an adequate and valid interpretation of data regarding the environmental fate and behavior of nanomaterials, it is essential to describe parameters like dissolution, dispersibility, and dispersion stability. The aim of the project was to develop a standardized test method to determine dispersibility and dispersion stability of nanomaterials in simulated environmental media as a new Organization for Economic Cooperation and Development (OECD) Test Guideline. For this objective, both conceptual and experimental work was conducted, and an international round robin was initiated and executed to validate the proposed test setup regarding reliability and reproducibility. According to UBA, two comprehensive expert commenting rounds of the OECD Test Guideline Program supported the refinement of the draft within this project. The test method developed was submitted to OECD by UBA and published by OECD in October 2017 as a new “Test Guideline on dispersion stability of nanomaterials in simulated environmental media (OECD No. 318).” The final report describes the considerations, experimental approaches, and obtained results that build the basis for the new OECD Test Guideline. The report also contains a literature review of transformation reactions of nanomaterials in environmental media (up to 2015).