Engineered nanomaterials (ENMs) are incorporated into numerous industrial, clinical, food, and consumer products and a significant body of evidence is now available on their toxicity to aquatic organisms. Environmental ENM concentrations are difficult to quantify, but production and release estimates suggest wastewater treatment plant effluent levels ranging from 10 to >10μgL for the most common formulations by production volume. Bioavailability and ENM toxicity are heavily influenced by water quality parameters and the physicochemical properties and resulting colloidal behaviour of the particular ENM formulation. ENMs generally induce only mild acute toxicity to most adult fish and crustaceans under environmentally relevant exposure scenarios; however, sensitivity may be considerably higher for certain species and life stages. In adult animals, aquatic ENM exposure often irritates respiratory and digestive epithelia and causes oxidative stress, which can be associated with cardiovascular dysfunction and the activation of immune responses. Direct interactions between ENMs (or their dissolution products) and proteins can also lead to ionoregulatory stress and/or developmental toxicity. Chronic and developmental toxicity have been noted for several common ENMs (e.g. TiO, Ag), however more data is necessary to accurately characterize long term ecological risks. The bioavailability of ENMs should be limited in saline waters but toxicity has been observed in marine animals, highlighting a need for more study on possible impacts in estuarine and coastal systems. Nano-enabled advancements in industrial processes like water treatment and remediation could provide significant net benefits to the environment and will likely temper the relatively modest impacts of incidental ENM release and exposure.