A review with 23 refs. Deterioration of polymeric materials is caused by adhering microorganisms that colonize their surfaces, forming biofilms; thus, biodeterioration is a biofilm problem. Biofilms consist of microorganisms embedded in a polymer matrix, attached to surfaces of all kinds. The structure and function of synthetic polymeric materials can be damaged by biofilms in various ways, among these: (1) by coating the surface, masking surface properties and contaminating adjacent media such as water by released microorganisms; (2) by increasing the leaching of additives and monomers out of the polymer matrix by microbial degrdn.; (3) by attack by enzymes or radicals of biol. origin to polymer and additives; leading to both embrittlement and loss of mech. stability; (4) by accumulating water and penetrating the polymer matrix with microbial filaments, causing swelling and increased cond.; and (5) by excretion of lipophilic microbial pigments that lead to unwanted colors in the polymer. Direct and indirect damage as caused by these five mechanisms can be very high.