Biofilms occur in a wide range of habitats were bacteria attach to surfaces and grow. In natural environments, biofilm populations are probably the prevalent form of microbial life on earth. Compared to the activated sludge process, one advantage of biofilms is the separation of the sludge age from the hydraulic retention time of the substrates. Since the growth rates of many specialised organisms are low, the growth within biofilms provides protection against washout. Even substrates which are difficult to degrade can be decomposed successfully in biofilms. Additionally, the degradation of many xenobiotics requires the combined action of several species present in bacterial communities. This metabolic cooperation successfully generates only when bacterial aggregates in the form of biofilms, sludge granules or flocs are present. Moreover, biofilm technologies enable the combined action of microbial degradation and non biological processes such as adsorbtion or catalytic reaction. By the use of such combined processes, even substrates which are toxic or non degradable by conventional microbial treatments, can be mineralised as well. A dominant factor for the biofilm treatment systems is the biofilm support. It provides the surface for the establishment of specialised biofilms and can affect the degradation properties of the overall treatment system. Moreover, several degradation steps can be realised by a catalytically acting carrier.