Conductivity measurement has been tried and tested for many years and, after pH measurement, is the most frequently used electrochemical measurement method in industrial measurement technology. It has been used in practice for over 100 years. Conductivity measurement is essential for monitoring the quality of water for pharmaceutical purposes. Real-time, inline measurements provide constant information on conductivity and temperature. Permanent data recording is thus possible and ultimately serves quality assurance. Compared to offline measurement, inline measurement is simpler and more cost-effective and helps to avoid errors in sampling, handling and transport. In addition to the theoretical basics, the underlying measuring principle, the influence of temperature and the corresponding temperature compensation will be explained in this lecture. The pharmacopoeia's requirements for metrology, be they limits or requirements, as well as the latest developments in the form of digital sensors are also covered.