A review with 61 refs. Microorganisms in aggregates such as flocs, film, and sludge do not only display biochem. and biol., but also phys. and physico-chem. properties. Among these are mech. stability, binding of water, diffusion, sorption, mass transport and optical properties, and friction resistance. These properties are chiefly caused by the extracellular polymeric substances (EPS) which fill the space between the cells and account for a considerable proportion of the org. carbon content of biofilms. The EPS consist not only of polysaccharides but also of considerable amts. of protein; nucleic acids and lipids are also found in the EPS. Above all, the EPS form the morphol. and internal structure of biofilms, including surface pores and channels. The EPS provide a matrix which allows the cells to maintain their position for a much longer period of time compared to the planktonic mode. This facilitates the formation of synergistic microconsortia of different species which can perform orchestrated degrdn. processes. Mech. stability of biofilms includes aspects such as sloughing of the biomass in biofilm reactors, resulting in possibly adverse effects to the process. On the other hand, when biofilms have to be removed as biofouling layers, it is the cohesive and adhesive forces which have to be overcome. Three types of weak interactions have to be considered, viz. hydrogen bonds, electrostatic interactions, and van der Waals interactions. As EPS contain many groups capable of different forms of these interactions, the binding force between macromols. is multiplied by the no. of interacting groups, which increases the overall binding force by several orders of magnitude. Interactions between extracellular enzymes and polysaccharides are known which stabilize the enzymes and possibly enhance their activity.