In the Netherlands, all drinking water prepared from groundwater is treated and distributed without the use of a chemical disinfectant. Surface water, which is used in about one third of the water supplies, must pass through multiple barriers for the removal of microorganisms and pollutants. A disinfection stage is included for most surface water that does not undergo soil passage but drinking water is distributed without or with a low disinfectant residual. Recontamination of drinking water is prevented by a series of measures including the prevention of backsiphonage, controlled use of fire hydrants, cleaning and repair protocols, and monitoring. Regrowth is controlled by maintaining biologically-stable drinking water, by the use of biologically-stable materials in contact with the water and by conducting a regular flushing program. A number of techniques are used to evaluate the biological stability of drinking water including the assessment of the concentration of easily available assimilable organic carbon (AOC), and biofilm formation characteristics of drinking water (biofilm formation rate and biofilm formation potential). These techniques have been applied in a number of water supply systems to obtain data for defining the biological stability of drinking water. This paper describes the results of such an investigation of the water supply of the City of Leiden.