A review. Extracellular polymeric substances (EPS) are biol. polymers of microbial origin, in most cases polysaccharides and proteins, but also nucleic acids, lipids etc. EPS form a highly hydrated gel that keeps microorganisms together. Thus, EPS mediate the formation of cell aggregates (e. g. biofilms and flocs), which represent the common form of life microorganisms in natural aquatic environments and are ubiquitous in man-made water systems. The EPS form and influence the space between the cells; they are mainly responsible for the biol. and physico-chem. properties of biofilms, flocs, and sludges. They bind water and form a matrix of a certain mech. stability, which allows the formation of stable microconsortia facilitating coordinated degrdn. processes, lead to the concn. and storage of nutrients and extracellular enzymes, and promote the interactions between different EPS components. Embedded in the EPS matrix, biofilm organisms can tolerate much higher concns. of toxic substances than planktonic organisms. There are indications that the EPS structure is not only a random accumulation of biopolymers but carries elements of organization and provides significant ecol. advantages for the inhabitants of microbial aggregates.