The US-EU Community of Research (CoR) was established in 2012 to provide a platform for scientists to develop a shared repertoire of protocols and methods to overcome nanoEHS research gaps and barriers (www.us-eu.org/
). Based on work within the Ecotoxicology CoR (2012-2015) we provide here an overview of the state-of-the-art of nanomaterials (NMs) in the aquatic environment by addressing different research questions with a focus on ecotoxicological test systems and the challenges faced when assessing nanomaterial (NM) hazards (e.g., uptake routes, bioaccumulation, toxicity, test protocols and model organisms). Our recommendation is to place particular importance on studying the ecological effects of aged/weathered NMs, as-manufactured NMs, as well as NMs released from consumer products in addressing the following overarching research topics: i) NM characterization and quantification in environmental and biological matrices, ii) NM transformation in the environment and consequences for bioavailability and toxicity, iii) alternative methods to assess exposure, iv) influence of exposure scenarios on bioavailability and toxicity, v) development of more environmentally realistic bioassays and vi) uptake, internal distribution, and depuration of NMs. Research addressing these key topics will reduce uncertainty in ecological risk assessment and support the sustainable development of nanotechnology.