A review on the compn., properties and functions of extracellular polymeric substances (EPS), esp. their role in microbial aggregation, with 130 refs. The EPS-mediated mech. stability of biofilms and other microbial aggregates is of importance for the performance of tech. biofilm processes as in wastewater treatment, but also in the control or removal of unwanted biofilms in other industrial systems. From an ecol. point of view, the EPS matrix provides the possibility that the microorganisms can form stable aggregates of mixed populations, leading to the synergistic microconsortia. This facilitates the sequential degrdn. of substances that are not readily biodegradable.