There has been growing concern about antimicrobial resistance for some years now. A vast amount of literature is available on the emergence of resistance and the use of antimicrobials in medicine, veterinary medicine and animal husbandry. Resistance genes and resistant bacteria have been detected in environmental compartments such as sewage, surface water, oceans, sediments, sewage sludge and soil (for a more detailed review of antibiotic resistance in the environment see Kümmerer, to be published). The most prominent medical examples are vancomycin resistant enterococci (VRE), methicillin resistant Staphylococcus aureus (MRSA), and multi-resistant pseudomonades. The selection pressure due to the presence of antibiotics above a certain concentration against the microbial biocoenosis is an important factor in the selection and spread of resistant bacteria. Transfer of resistance genes as well as the already resistant bacteria themselves is favored particularly by the presence of antibiotics over a long period and in sub-therapeutic concentrations. Exposure of bacteria to such sub-therapeutic antimicrobial concentrations is thought to increase the speed with which resistant bacterial strains are selected, e.g. if antibiotics are used as growth promoters or by improper use in veterinary medicine and medicine. The development of resistance through the input of antibiotics into the environment is a new issue in this discussion.