Interdisciplinarity is of first importance to evaluate the risks associated with nanotechnology. The reasons are that nanomaterials are very new materials that combine nano-sizes and new reactivities. The complexity comes from the very low concentrations of nanomaterials in the environmental medium, the transformations of the nanomaterials due to the reactivity of the surface, the transfer in the environmental media, particularly in the presence of liquid water (soils, sediments, surface water), which implies an association with natural colloids (organic or minerals) and blockage in some compartments. These properties govern the hazard that strongly depends on exposure and speciation.