The growing world population, in conjunction with climate change, cases the global water de-mand to increase. It is, therefore, crucial to protect existing water resources from chemical pollution and contamination by pathogens. An increasingly popular strategy to meet increased water demand is pota-ble water reuse, which uses wastewater effluent as the water source and treats it to such an extent that it is safe to drink. However, this practise may encompass risks for human health if the treatment train does not ensure a sufficient removal of chemical and microbial contaminants. In the context of both wa-ter and wastewater treatment, ozonation has emerged as an efficient treatment method for the control of pathogens, the abatement of organic micropollutants, and the removal of taste and odor compounds. Despite its popularity, information on the inactivation of waterborne viruses by ozone is scarce. The aim of this thesis was to investigate the kinetics and mechanisms of inactivation of water-borne viruses by ozone. In a first step, the inactivation kinetics of a suite of human viruses and commonly used surrogates (bacteriophages) were determined in well-controlled buffer systems. To this end, we developed a method to control O3 decay in batch reactors. This allowed us to measure virus inactivation as a function of low ozone exposures. The resulting inactivation rate constant (kO3-virus) differed between the virus species studied, but all kO3-virus fell within a narrow range of 105-106 M-1s-1. These high inactivation rate constant indicate that ozone is efficient to inactivate waterborne virus. Increases in temperature and pH resulted in an increased kO3-virus, though the effect was relatively minor. To determine if more complex, natural water matrices influence virus inactivation by ozone, we investigated the virucidal efficacy of ozone in two surface waters and a secondary wastewater effluent. While inactivation kinetics as a function of ozone exposure initially corresponded well to those observed in buffer solutions, the inactivation curve tailed off at higher ozone exposures. Furthermore, because it is not possible to measure virus inactivation during water treatment in real-time, we tested different if “easy-to-measure” proxies can be used to track virus inactivation. We determined that the applied specif-ic ozone dose, the reduction in UV254, or the abatement of carbamazepine all correlated to virus inactiva-tion, though some proxy-inactivation relationships were not universal but depended on the water type. The proxies were validated in a pilot-scale ozonation reactor treating Lake Zurich water, and were found to provide good estimates of virus inactivation. Finally, an immunostaining assay was developed to observe how ozonation affects crucial steps in the life cycle of echovirus 11, a representative of the Enterovirus genus. Specifically, we investigated if ozone alters the ability of the virus to enter the host cells, and if it prevents the viral genome from replicating. Preliminary results indicate a loss of internalization after ozone treatment as well as genome replication. In conclusion, this thesis provides new information on the efficacy and mechanisms of ozone as an inacti-vating treatment for waterborne viruses, and it delivers tools to monitor virus inactivation during water and wastewater treatment in real-time.