The contamination of surface waters with organic micropollutants is a well known problem and increased over the last decades. One reason for their release and consequently their detection in surface waters is their incomplete degradation and therefore insufficient removal during conventional waste water treatment processes. To overcome this problem advanced oxidation processes have been proposed as an additional treatment step, since the formation of highly reactive hydroxyl radicals helps to degrade those substances. This study was therefore designed to investigate the toxicological properties (cytotoxic, genotoxic, mutagenic and estrogenic) of several substances before and after ozonation or UV/H2O2 treatment in either waste water treatment plant effluents or HPLC-water using toxicological in vitro methods. These methods are able to identify biological effects of the whole water sample thus the entire complexity of chemicals. In addition cellular based methods will give an overview on possible induction mechanisms which will lead to a manifestation of effects on the organ or even organism level. In the case of Terbutryn its genotoxicity before treatment was first increased at low ozone dosages (36, 100, and 195 μg/L) but was removed at the highest ozone dosage (800 μg). Similar results have been shown for TPP. Genotoxic effects were only seen before ozonation, but after 60 minutes of ozonation these effects were removed. The ozonation of Triclosan resulted in the formation of the by-product 2,4-Dichlorophenol. Both substances did not exhibit cytotoxic effects up to a maximum tested concentration of 100 μg/L. Genotoxic effects were only seen for Triclosan starting at a concentration of 10 μg/L. Thus a less genotoxic by-product has been formed during ozonation. The estrogenicity tests showed that HPLC-water containing Bisphenol A did no longer have estrogenic effects after UV/H2O2 treatment whereas neither the ozonation nor the UV/H2O2 treatment of Ethinylestradiol for 60 minutes resulted in a 28 complete loss of estrogenic activity. The musk fragrance HHCB in contrast did not show estrogenic effects, neither before nor after ozonation. For the two beta blockers (Atenolol and Metoprolol), the antibiotics Sulfamethoxazole and Ofloxacin as well as the musk fragrance AHTN neither before nor after ozonation or UV/H2O2 oxidation cytotoxic effects were detected. The ozonation of Irgaol 1051 did also not lead to cyto- or genotoxic effects. The results of the oxidation experiments with Bisphenol A, Ciprofloxacin and HHCB demonstrated that formed oxidation by-products differ in regard to toxicity depending on treatment method and water matrix. Cytotoxic effects for all three substances were only detected in HPLC-water after 60 minutes of UV/H2O2 but not in waste water treatment plant effluent. In addition no toxicity was detected after the same time of oxidation using ozone. The tested concentrations of TCPP as well as TCEP were not toxic before oxidative treatment. However cytotoxic effects were detected after UV/H2O2 treatment whereas the ozonation did not induce cytotoxicity. In general it can be concluded that ozonation as well as UV/H2O2 oxidation are two useful methods for the removal of the here tested substances from the effluents of waste water treatment plants in regard to their degradation and to prevent the formation of toxic oxidation by-products considering the used test methods (MTT Test, PAN I, Alkaline Comet Assay, Ames Test, and ER Calux). However a generalized statement about micropollutants their removal and toxicity is not possible, and needs to be established on a case by case basis since operating conditions as well as the matrix composition vary over time. The successful use of toxicological in vitro methods for the detection of biological effects of waste water treatment plant effluents in combination with advanced oxidation processes is confirmed by other studies, e.g. a study performed by the Eawag at the waste water treatment plant in Regensdorf or the PILLS project.