A review. Extracellular polymeric substances (EPS) are biopolymers of microbial origin and consist mainly of polysaccharides, proteins, lipids and nucleic acids. The EPS mediate adhesion to surfaces and form a hydrogel matrix for biofilms and other microbial aggregates. This matrix can be considered as \"house\" of the microorganisms which allows for the formation of stable communities (\"microconsortia\") of synergistic strains and enables them to degrade recalcitrant substances. EPS retain water and prevent desiccation. Cohesion and adhesion are provided by a network of fluctuating adhesion points. External pressure can change the structure from a gel to a highly viscous liq. Due to their sorptive properties, dissolved nutrients from the water phase are accumulated and increase the survival chances of biofilm organisms in oligotrophic environments. The matrix facilitates gene exchange and regulation processes via signaling mols. It provides a template for extracellular enzymes and prevents that they are washed out. Thus, it is of great importance for the degrdn. of solids and particles. Remnants of lysed cells are retained and can be utilized as food source. Thus, biofilms can be considered as a natural example for sustainable use of nutrients. Some EPS are biotechnol. employed as additives for food, drilling fluids and as biosurfactants.