Electrochemical water treatment is a simple method to generate disinfecting agents. Several companies offer special cell technologies for water disinfection. It has been found that besides active chlorine for disinfection, by-products such as chlorate and perchlorate may be formed. Systematic studies using laboratory cells, semi-technical and technical cells confirm this. Mixed oxide and boron doped diamond anodes were used in laboratory-scale and semi-technical experiments which were conducted under drinking water conditions at temperatures between 10 and 30°C and at current densities between 50 and 500 A m-2. The results of these studies show a perchlorate formation potential. This book presents and discusses current research in the study of perchlorate formation in electrochemical water disinfection.