Pharmaceuticals are bioactive chemicals that are mostly released to the environment via municipal wastewater treatment plants. Although they are widely detected in surface waters, the environmental fate and exposure of biota are still largely unknown. The results in this thesis show that microbial transformation was more efficient under aerobic than anaerobic conditions, however large differences were found between three pharmaceuticals studied experimentally. Diclofenac was recalcitrant, bisoprolol partially biotransformed and naproxen readily biodegradable