Ozone is a strong oxidizing agent that is able to kill microorganisms particularly effectively and thus opens up the possibility of disinfecting water safely and effectively. Ozone has been used for drinking water treatment for over 130 years. Ozonation is an effective method with an increasing trend for the sanitization of ultrapure water in pharmaceutical and semiconductor production as well as other treated waters. However, ozone is also a highly toxic gas for humans and poses a health risk at high concentrations. To ensure that ozone concentrations remain within safe limits during cold sanitization and that ozone overdoses do not occur, ozone measurements should be carried out regularly. In this lecture, in addition to the theoretical basics, the generation of ozone, the possible applications and the correct installation in the water system, the technology behind the ozone sensors will also be examined.