Several works based on the use of UV–visible spectrophotometry for natural water quality monitoring were published in recent years. Not only rivers, lakes, marshes, and wetlands but also groundwater and seawater were chosen as experimental sites in numerous studies since the first UV application for water examination 60 years ago. From our side, we have carried out several experiments on different water bodies, both in France and in Quebec, and further UV spectra of water samples from Wales were studied with some of our scientific partners in the frame of research projects. The study of discharges (wastewater or accidental spills) in freshwaters is also facilitated by this technique. Given a new chapter in this edition dealing with remote sensing and high-frequency monitoring, experiments using these techniques are not reported in the present chapter