Vulnerability of Water Distribution Systems to Leakage is defined using standard terminology. The use of Water Balance Method illustrates a basic approach to leakage. Moreover, the suitability of various performance indicators is discussed and illustrated using both technical as well as financial data. Methodology to evaluate leakage (e.g. DMA, SCADA) provides the operators with a suitable means how to decrease the leakage. Field measurements of hydraulic parameters and mathematical modelling of Water Distribution Systems may substantially help to quantify leakage and to evaluate the best practice for integrated leakage reduction.