Dr. Petersen is recognized for his leadership in developing an Organization for Economic Cooperation and Development (OECD) guidance document on ecotoxicity testing of manufactured nanomaterials (MNs). Existing OECD ecotoxicity test methods were not designed for MNs. Uncertainty in their application to MNs has hindered commercialization of advanced MN-enabled products. With Dr. Petersen's leadership during a six-year process, international consensus was reached on recommendations related to key issues, leading to an OECD guidance document published in 2020. The detailed recommendations underpin the use of regulatory ecotoxicity testing methods for MNs, enabling manufacturers to perform consistent ecotoxicity testing of MNs with confidence that results will be accepted globally.