The subject of this paper are water losses in the water supply system, as a threatening factor to the continuous and safe water supply as well as sustainable operation of the relevant water company. After basic introduction with the structure of water losses and their causes, it becomes evident that crucial influence on their appearance and amount of lost water have management and maintenance of the water supply system together with the development policy of the company. The domestic water companies often operate on the breakeven point, without sufficient technical staff which would in a good, safe and sustainable way managed the system, what results in unsustainable operative conditions, with unbalanced pressure zones. Due to the lack of available financial resources, maintenance is often reduced to the urgent technical repairs, which ultimately results in a high rate of water losses in the system. Considering that each cubic meter (m3) of „lost“ water represents the financial loss for the company, this situation certainly leads to uneconomical system management. Also, water losses threaten the regular water supply, cause damage to the surrounding infrastructure and require more energy than necessary which certainly has a negative environmental influence. This paper provides a theoretical overview on the methodology of calculation and representation of water losses, detection methods of their initial causes, as well as practical example of current state evaluation of the water supply in the settlements of Pirovac, Tisno and Jezera, based on the presented methodology.