The emerging literature on the ecotoxicity of nanoparticles and nanomaterials is summarised, then the fundamental physico-chemistry that governs particle behaviour is explained in an ecotoxicological context. Techniques for measuring nanoparticles in various biological and chemical matrices are also outlined. The emerging ecotoxicological literature shows toxic effects on fish and invertebrates, often at low mg l(-1) concentrations of nanoparticles. However, data on bacteria, plants, and terrestrial species are particularly lacking at present. Initial data suggest that at least some manufactured nanoparticles may interact with other contaminants, influencing their ecotoxicity. Particle behaviour is influenced by particle size, shape, surface charge, and the presence of other materials in the environment. Nanoparticles tend to aggregate in hard water and seawater, and are greatly influenced by the specific type of organic matter or other natural particles (colloids) present in freshwater. The state of dispersion will alter ecotoxicity, but many abiotic factors that influence this, such as pH, salinity, and the presence of organic matter remain to be systematically investigated as part of ecotoxicological studies. Concentrations of manufactured nanoparticles have rarely been measured in the environment to date. Various techniques are available to characterise nanoparticles for exposure and dosimetry, although each of these methods has advantages and disadvantages for the ecotoxicologist. We conclude with a consideration of implications for environmental risk assessment of manufactured nanoparticles.