In contact with atmosphere, soil and rocks natural waters exchange numerous components, such as gases, solid particles, ions and organic matter. These exchange processses are mostly very complex and numerous equilibria are the result of theses processes. After an introduction to these processes, the author discusses the kind and nature of the components and dissolved substances which are found in rivers, lakes, groundwater and soil water. Biological, photochemical and climatic interactions which influence water composition are discussed in detail with special emphasis on processes taking place in the saturated and unsaturated zones of the soil. More and more, anthropogenic substances are observed to enter natural waters and for this reason theses substances receive critical attention. A comprehensive chapter discusses the mass-balance of substances in the water and treats their sources and sinks.