Trihalomethanes (THMs) are produced when certain organic materials are treated with chlorine. The U.S. EPA recently has promulgated a Maximum Contaminant Level for THMs in finished drinking water of 0.10 mg/L. Although ozone will not oxidize THMs at any significant rate once they have been formed, its use has been studied at many North American water treatment plants for oxidation of THM precursors before chlorine is added. In most cases ozonation prior to chlorination lowers THM formation potential (THMFP), but there have been many reports of increased THM formation when ozonation is followed by chlorination. The known chemistry of THM formation and of ozone oxidation of organic materials to both destroy and to form THM precursors is discussed to explain these results. In addition, recommendations are made for the use of ozone at various points in the standard drinking water treatment process, by which THM levels in the finished water can be decreased. These methods include ozonation at any point prior to chlorination and/or prior to a biological treatment process, such as adsorption of organics by means of granular activated carbon or by passage through slow sand filters. Pretreatment of raw waters to remove as much organic material as possible before adding any chemical oxidant, particularly chlorine, remains the best initial approach to controlling THMs, as well as other undesired by-products of oxidation processes.