Ultraviolet light in the UV-C band (200–280 nm) is also known as ultraviolet germicidal radiation and is widely used for sterilizing equipment and creating sterile environments, as well as in the food and the water industry to inactivate microorganisms. UV-C radiation has been used to treat water since 1909 and offers a safe and effective disinfection process for drinking water. Waterborne pathogens in drinking water have different sensitivities to UV exposure, being greatest in bacteria and protozoa > viruses > bacterial spores > Adenovirus (an exception as it is the only virus with low sensitivity to UV) > algae, which are least sensitive. The history of UV sterilization and its action on the DNA or RNA of microorganisms is explained. Different UV lamps are considered as is the process technology used. However, unlike chlorine and its derivatives, UV-C does not offer any residual disinfection capability leaving supplies vulnerable to microbial contamination during distribution and within the consumer’s home. Therefore, its high oxidizing capability is often used prior to chlorination to ensure high microbial quality water as well as significantly reducing the chlorine dosage required. While there is no direct disinfection by-product formation, photonitration can lead to the production of a range of new nitroorganics during UV treatment that can form halonitromethanes if the water is subsequently chlorinated. The technology is particularly useful for households supplied via boreholes, springs and surface waters as a point-of-use or point-of-entry system; however, pre-filtration is advised to remove fine particulates that could offer protection to pathogens. It is also widely used in high-rise commercial and residential buildings, as well as hospitals, to ensure that stored water pumped to the service area at the top of the buildings, often at night, is disinfected before subsequently being used. It is increasingly used in conjunction with shower systems to protect against Legionnaires’ disease.