This paper considers whether current standard ecotoxicity methods are fit for purpose for assessing the hazards of engineered nanoparticles. We conclude that the types of test species and biological endpoints used within standard environmental hazard assessment frameworks are generally appropriate. However, there are areas of considerable uncertainty associated with characterisation of nanoparticle exposure in test systems that apply to all ecotoxicity testing guidelines, except those in which dosing of nanoparticles is oral. These include the way in which the substance is dosed into, and maintained within, the test medium; measurement and characterisation of nanoparticles in the test system; better understanding and reporting of abiotic factors that influence behaviour of nanoparticles in the test medium; and agreement on how dosimetric data should be reported.