Introducing the "Nano-world" -- Nanoparticles Acting as Condensation Nuclei - Water Droplet Formation and Incorporation -- Nanoparticles in Groundwater - Occurrence and Applications -- Composition and Transport Behavior of Soil Nanocolloids in Natural Porous Media -- Removal of Organic and Inorganic Pollutants and Pathogens from Wastewater and Drinking Water Using Nanoparticles - A Review -- Adsorption/Desorption Behavior of Charged Polymer Nanoparticles on a Mineral Surface in an Aqueous Environment -- X-Ray Spectromicroscopy Studies of Nanoparticles in Aqueous Media -- In Situ Measurements on Suspended Nanoparticles with Visible Laser Light, Infrared Light, and X-Rays -- Coupling Techniques to Quantify Nanoparticles and to Characterize Their Interactions with Water Constituents -- Nanoparticles: Interaction with Microorganisms -- Ecotoxicology of Engineered Nanoparticles -- Standardisation. As nanotechnology enters everyday life, engineered nanoparticles (ENP) will find their way into nature, including surface and groundwater. Here, distinguished experts of water chemistry present dedicated methods for the analysis of nanoparticles in the aquatic environment, their distribution and fate. This includes the influence of complex matrices such as wastewater, brown water with natural organic matter (NOM), and high salt concentrations as well as available and future standardized methods. The background of geogenic, natural nanoparticles is considered in a discussion of known environmental effects, including strategies to test for potential effects on human and environmental health.