When water leaves a treatment works and travels through a distribution system, its quality, with respect to many chemical and biological parameters, will degrade. The quality of the delivered water will be largely influenced by: -The quality of treated water supplied into the network -The condition of distribution assets within the network -The retention time within the network. The water industry has focused predominantly on the quality of treated water and the physical condition of distribution assets when improving the quality of water at the customer's tap. However the quality of the water delivered is also affected by the time the water is retained in the different elements of the distribution network. Retention time is controlled both by the physical characteristics of the system and the operational regime. Physical characteristics such as pipe roughness may change throughout the life of the asset or be modified by rehabilitation. Operational activities may be structured; for example, pump scheduling and planned maintenance, or uncontrolled as in the case of demand driven operational responses. Changes to water quality result from the reactions in the bulk water with time and through the chemical and biological reactions with the distribution system materials with which The chemical and biological reactions in the bulk water are relatively well understood enabling development of models and software for predicting changes. The reactions of the water at the pipe-water interface are more complex and less well understood. The aim of this research is to demonstrate that water quality within distribution networks can be managed effectively by controlling retention time and to develop practical and pragmatic methodologies for doing so.