Engineered nanomaterials (ENMs), intentionally synthesized materials with sizes less than 100 nm in at least one dimension, have numerous potential environmental applications, such as pollution remediation and water treatment. However, concerns regarding their potential health and environmental impacts have been raised. In this Review, we assess the opportunities of ENMs in environmental applications versus their potential public and environmental health risks, focusing on water treatment and reuse, and identify strategies for their responsible use. Life-cycle analyses indicate that the highest potential environmental and health impacts of ENMs used in commercial products are associated with production rather than incidental release during use. Typically, the detected or predicted ENM concentrations are 1 to 4 orders of magnitude lower than their respective predicted no-effect concentrations. In addition, ENMs often undergo passivating transformations, such as agglomeration and oxidation, reducing risks after release. Therefore, the environmental and health risks of ENMs are relatively low. However, some point sources under extreme scenarios, such as sewage effluent, can potentially increase localized risks. Adopting green chemistry and immobilization strategies can further limit the release of ENMs, minimizing their potential discharge into the environment. Such strategies to reduce toxicity and exposure enable sustainable application of ENMs, such that the environmental benefits could outweigh the risks if managed properly. Engineered nanomaterials (ENMs) have numerous environmental applications, such as in water treatment and reuse. This Review explores the trade-offs between the risks and benefits of environmental ENMs, and highlights that the environmental and health risks of ENMs are relatively low when used responsibly. Environmental applications of nanomaterials include pollution control, green chemistry, clean water production, and sensing and monitoring.Despite the potentially substantial environmental benefits of nanotechnology, the large-scale manufacturing requirements, cost limitations and potential health and environmental risks of engineered nanomaterials (ENMs) are common barriers to their widespread use.The environmental and health risks of ENMs are relatively low considering the very low ENM concentrations involved and the passivating transformations that occur in the environment, although the potential human health and ecosystem impacts of long-term (months to years) exposure to low ENM concentrations (for example, sub-microgram per litre level in water) remain largely unexplored.Life-cycle analyses of ENMs used in commercial products indicate that the highest potential environmental and human health risks are associated with production rather than incidental release.To prevent their release into the environment and mitigate exposure, ENMs should be immobilized in or on substrates such as electrodes, membranes and other matrices. Immobilization also enables ENMs to be reused, promoting sustainable and circular practice.Additionally, ENM-enabled products and processes should undergo a certification process to meet regulated safety standards to promote best practice and increase social acceptance.