NanoReTox will identify the potential risks to the environment and human health posed by free engineered (i.e. manmade) nanomaterial by comprehensively addressing five key questions: (1) How does the environment into which nanoparticles are released affect their physicochemical properties and their bioreactivity? (2) How does this impact on their ability to interact with and/or penetrate mammalian and aquatic cells and organisms (bioavailability) and will bioavailability result in toxicity? (3) Is there a pattern of cellular reactivity and/or toxicity related to physicochemical properties, i.e. a hierarchy of activity? (4) What combination of conditions discovered in (1-3) above are most likely to pose a risk to human health and the environment? (5) How can this information be incorporated in a risk assessment model? We have assembled a team of experts from across the EU and the US whose combined expertise can address these questions in depth, and therefore comprehensively cover the scope of research topic NMP-2007-1.3-2 – Risk assessment of engineered nanoparticles on health and the environment.