Nanotechnology is expected to contribute to the protection of the environment, but many uncertainties exist regarding the environmental and human implications of manufactured nanomaterials (MNMs). Contradictory results have been reported for their ecotoxicity to aquatic organisms, which constitute one of the most important pathways for their entrance and transfer throughout the food web. The present review is focused on the international strategies that are laying the foundations of the ecotoxicological assessment of MNMs. Specific advice is provided on the preparation of MNM dispersions in the culture media of the organisms, which is considered a key factor to overcome the limitations in the standardization of the test methodologies.