The article summarizes the ecotoxicology and chemistry of manufactured nanoparticles (NPs). It discusses the ecotoxicological effects of NPs on fish and invertebrates, often observed at low concentrations. However, data on bacteria, plants, and terrestrial species are limited. Initial data suggest that NPs may interact with other contaminants, influencing their ecotoxicity. Particle behavior is influenced by size, shape, surface charge, and the presence of other materials. NPs tend to aggregate in hard water and seawater, and are influenced by organic matter or colloids in freshwater. The state of dispersion affects ecotoxicity, but factors like pH, salinity, and organic matter remain to be systematically studied. Environmental concentrations of NPs are rarely measured. Various techniques are available to characterize NPs for exposure and dosimetry, but each has advantages and disadvantages. The article concludes with implications for environmental risk assessment of NPs. Nanotechnology is a new area of science and technology with a global market estimated at $10.5 billion in 2006. Nanotechnology involves the intentional generation or modification of materials at the nanometer scale. Although nano-scale materials have been used in materials research for a decade, there is now a wider debate about the risks and benefits of manufactured nanomaterials and consumer products. The benefits of nanomaterials are potentially enormous, with applications in electronics, optics, textiles, medical devices, cosmetics, food packaging, water treatment, fuel cells, catalysts, biosensors, and environmental remediation. Environmental concentrations of NPs have not been routinely measured, but there are concerns that NPs may be released from products over their life or that product applications could generate wastes containing nanomaterials. It is unclear whether sewage treatment works could completely remove NPs from final effluents. There is a concern that these novel materials could be released into the environment, and that there may be releases from products in current use. However, we are only just starting to explore their ecotoxicology and environmental chemistry. The environment contains many natural particles at the nm scale, such as colloids in freshwater, volcanic dusts in the atmosphere, and nm scale particles from soil erosion. These materials have been in the environment for millions of years, and organisms must be adapted to living in the presence of these natural substances. There are also concerns that anthropogenic activity has been generating nano-scale pollutants, such as air-borne particles from car exhausts or nanoparticles generated from the erosion of materials such as car tyres. Even so, we still have much to learn about the fate and behavior of natural colloids and their interactions with pollutants. Manufactured NPs might represent a special case, since they may be designed to have particular surface properties and (surface) chemistries that are less likely to be found in natural particles. They might therefore present enhanced or novel physico-chemical or toxicological properties in comparison to natural NPs.The article summarizes the ecotoxicology and chemistry of manufactured nanoparticles (NPs). It discusses the ecotoxicological effects of NPs on fish and invertebrates, often observed at low concentrations. However, data on bacteria, plants, and terrestrial species are limited. Initial data suggest that NPs may interact with other contaminants, influencing their ecotoxicity. Particle behavior is influenced by size, shape, surface charge, and the presence of other materials. NPs tend to aggregate in hard water and seawater, and are influenced by organic matter or colloids in freshwater. The state of dispersion affects ecotoxicity, but factors like pH, salinity, and organic matter remain to be systematically studied. Environmental concentrations of NPs are rarely measured. Various techniques are available to characterize NPs for exposure and dosimetry, but each has advantages and disadvantages. The article concludes with implications for environmental risk assessment of NPs. Nanotechnology is a new area of science and technology with a global market estimated at $10.5 billion in 2006. Nanotechnology involves the intentional generation or modification of materials at the nanometer scale. Although nano-scale materials have been used in materials research for a decade, there is now a wider debate about the risks and benefits of manufactured nanomaterials and consumer products. The benefits of nanomaterials are potentially enormous, with applications in electronics, optics, textiles, medical devices, cosmetics, food packaging, water treatment, fuel cells, catalysts, biosensors, and environmental remediation. Environmental concentrations of NPs have not been routinely measured, but there are concerns that NPs may be released from products over their life or that product applications could generate wastes containing nanomaterials. It is unclear whether sewage treatment works could completely remove NPs from final effluents. There is a concern that these novel materials could be released into the environment, and that there may be releases from products in current use. However, we are only just starting to explore their ecotoxicology and environmental chemistry. The environment contains many natural particles at the nm scale, such as colloids in freshwater, volcanic dusts in the atmosphere, and nm scale particles from soil erosion. These materials have been in the environment for millions of years, and organisms must be adapted to living in the presence of these natural substances. There are also concerns that anthropogenic activity has been generating nano-scale pollutants, such as air-borne particles from car exhausts or nanoparticles generated from the erosion of materials such as car tyres. Even so, we still have much to learn about the fate and behavior of natural colloids and their interactions with pollutants. Manufactured NPs might represent a special case, since they may be designed to have particular surface properties and (surface) chemistries that are less likely to be found in natural particles. They might therefore present enhanced or novel physico-chemical or toxicological properties in comparison to natural NPs.