The rapid development of nanotechnology and the increased use of engineered nanoparticles (ENPs) in consumer products will result in the occurrence of nanomaterials in the environment. The present thesis (compilation of publications) is dealing with the investigation of ENP stability, fate and impact in aquatic systems. Influence of important water physico-chemical properties (pH, ionic strength, divalent electrolytes) but also the presence of natural organic matter (NOM) and surfactants on the stability of TiO2 ENPs has been systematically studied. A special interest was given to the stability of already formed ENP agglomerates. Presence of NOM, at environmental concentrations, was shown to promote significant disagglomeration which is a key result for the risk assessment associated to ENPs. Interactions mechanisms between ENPs and NOM were also investigated by calorimetry for the first time in environmental nanoscience and opened a new insight into the understanding of ENP behavior in presence of NOM.