Microbial layers on interfaces, called "biofilms" are characterized by a polymeric, highly hydrated gel matrix of microbial origin in which the organisms are embedded and immobilized. Biofilms exhibit degradation behaviour different from that free cells and accumulate water, metabolites, and sorbe substances at the interface. They can greatly interfere with technical processes by increasing the fluid friction factor, the hydrodynamic resistance of filter membranes and heat transfer resistance in heat exchanger systems. They can cover surfaces, impart unwanted colour, and contaminate water. Biofilms can attack their support and lead to microbially influenced corrosion (MIC). It is estimated that about 20 % to all corrosion damage to metals and building materials is due to microbial influence. Thus, the annual casts of biodeterioration range in billions of dollars. Biofilms play a key role in these processes. They consist of a gel matrix, formed by slime substances, in which the cells are immobilized on the surface of the material. Corrosion is an interfacial process, governed by ph, redox potential, oxygen concentration and other parameters. Biofilms, acting as a transport barrier, may change all these parameters exactly at the site where corrosion takes place compared to the water or air phase surrounding. Thus, a thorough knowledge of the properties and development of biofilms is crucial for the design of effective countermeasures.