Pharmaceutical residues are reported to be widely present in the aquatic environment. In terms of water protection, they represent a special group of substances. As so-called Emerging Environmental Substances they are under suspicion being toxic as well as disruptive to the endocrine system of organisms. Pharmaceuticals are relevant micropollutants in our waterways, which are usually delivered from the effluent of waste water sewage treatment plants (WWTP). During waste water treatment, pharmaceuticals can be removed from the water phase. Important mechanisms of removal are biological degradation, sorption and volatilization. This thesis focuses on the occurrence and retention of pharmaceuticals during waste water treatment. The aim of this work is to show which pharmaceuticals can be removed during biological waste water treatment. Nineteen selected pharmaceuticals and two metabolites of 6 indication classes were investigated in two Austrian municipal waste water treatment plants. Using the operational data of the treatment plants, mass balances of the pharmaceuticals could be done. Using the instrument of mass balancing, the behaviour of the selected substances could be calculated. Good biological degredation (> 90 % of the inlet load) in the biological treatment and the sludge digestion in both WWTP was identified for Ibuprofen, Cocain and its metabolite Benzoylecognin. Partly biological degradation (20 90 % of the inlet load) could be investigated for Clarythromycin, Erythromycin and Bisoprolol. Diclofenac, Phenazon, Roxithromycin, Metoprolol, Sotalol and Carbamazepin showed a degradation rate < 20 % of the inlet load. For all other selected pharmaceuticals the calculated results of WWTP A and B were not consistent. While the results for the biological degradation spread over a wide range, the removal via adsorption to the sludge is negligible. In total, the results of the pharmaceuticals mass balances are comparable with reports in literature.