Antibiotics are used extensively in human and veterinary medicine as well as in aquaculture for the purpose of preventing or treating microbial infections, while in livestock farming they are used to promote the growth of animals. Some antibiotics are also used in fruit growing and in bee keeping. Antibiotics are only partially eliminated in sewage treatment plants and residual amounts can reach surface waters, groundwater or sediments. In natural waters, most pharmaceuticals are only found in the μg l−1 range. Since biocidal substances are designed to cause a biological effect, when reaching the environment they may affect bacteria and other water and soil-dwelling organisms. It is only in recent years that a more complex investigation of antibiotic substances has been undertaken in order to permit an assessment of the environmental risks they may pose.