This document provides guidance for aquatic (including sediment) ecotoxicity testing of Manufactured Nanomaterials (MNs) to determine their hazard. The definition of MNs as having one dimension between 1 and 100 nm is generally adopted. However, the guidance provided here should also be relevant for colloidal (nano) particles (e.g., 1 to 1000 nm) with primary or aggregate/agglomerate sizes greater than the range for MNs. The guidance focuses on freshwater aquatic and sediment toxicity test methods described by the OECD Test Guidelines. More specifically, this guidance document addresses practical aspects of carrying out valid tests with MNs including interpreting and reporting the results, and addresses the specific issues. It also addresses modifications or additions to OECD TG procedures intended to incrementally improve the accuracy, intra-laboratory repeatability, inter-laboratory reproducibility and intra-laboratory reproducibility of test results. While the focus was specifically on adapting the OECD TGs for reliable MNs testing, the methods and principles discussed herein are likely more widely applicable to aquatic toxicity test methods published by other organisations.