Environmental Context. The fate and behaviour of trace pollutants are very strongly modified, and usually dominated, by their physical and chemical interactions with naturally occurring aquatic colloids (defined as solid phase material with one dimension between 1 nm and 1 µm). This review summarises the area and key advances in the field of natural aquatic colloids, including technique development and quantification of colloidal structure and interactions with pollutants. The review also discusses areas in which significant advances are likely to be made or are needed and, as such, provides a framework for further work in the next few years. Natural aquatic colloids are materials with one dimension between 1 nm and 1 µm. More informally defined, nanoparticles are materials with at least one dimension less than 100 nm. Both colloids and nanoparticles have significant effects on pollutant, nutrient, and pathogen chemistry, transport and bioavailability, and may themselves be bioavailable. Techniques for their fractionation, characterization and analysis have improved greatly in recent years. Although knowledge of their structure and environmental impact has also increased, it has not done so to the same degree and thus the field awaits the substantial application of new methodologies. This paper reviews the current state of the art in this area and also discusses likely future developments.