UV radiation is an important ecological factor. Amongst other effects it exhibits bactericidal activity with a maximum at a wavelength of 260 nm. UV interferes directly with the DNA structure in living cells. In response to increasing requirements for treated drinking water, UV-disinfection methods have become increasingly important due to the non-toxic nature of the disinfection products and to the efficient operation of low pressure Hg lamps. When compared with other important disinfectants, UV-methods are found to be more likely to meet the demands of modern drinking water disinfection. UV-methods offer distinct advantages with respect to the kinetics of disinfection and to the fact that no adverse effects of the radiation result in the treated water. Experiments carried out under operation conditions are described, demonstrating how UV-systems can be assessed and their limitations under variable conditions encountered in technical-scale operation.