The distribution of pollutants in water is strongly dependent on the processes at the solid-liquid interface, such as soprtion. The solid phase is represented by many different materials, mostly minerals but also metals and organics. In technical and natural water systems, biofilms occur to a greater or lesser extent. They will cover the underlying material (substratum) at least partially and represent a gel-type layer, mainly consisting of extracellular polymeric substances (EPS), formed by biofilm organisms, in which the latter are embedded. Thus, the sorption properties of biofilms will significantly determine the overall process. This effect may have been almost completely overseen, e.g., in the assessment of the fate of pollutants in waters, in the process of sediment diagenesis, and in chemical sorption studies. Sorption data can differ considerably between sterile and non-sterile assays; this aspect puts in question many results gained in biologically uncharacterized systems. Sorption in biofilms can be undesired, if pollutants accumulate in biomass such as activated sludge. The same effect, however, can be biotechnologically exploited, e.g. in biosorption reactors. Biofilms can sorb water, inorganic and organic solutes and particles. As sorption sites can serve: EPS, cell walls, cell membranes and cell cytoplasm. These sites display different sorption preferences, capacities and properties. The situation becomes even more complex as biofilms may respond physiologically to sorbed substances. For example: the uptake of toluene can lead to the formation of uronic acids in the EPS and, thus, to an increased sorption capacity for cations. When decomposing, biofilms will release sorbed substances. This can be of significance if trickling deposition of sewage water on soil is finished. The biomass will decompose, sorbed pollutants are remobilized and can contaminate the ground water if not retained abiotically by other soil components. A substantial research demand is identified regarding the following questions: (i) what do biofilms sorb; (ii) what are the binding sites; (iii) what are the sorption mechanisms and capacity, and (iv) what is the remobilization potential?