Assimilable organic carbon (AOC) is the fraction of carbon utilised by heterotrophic organisms, potentially leading to (re)growth, loss of biological stability and deterioration of water quality within drinking water distribution system (DWDS). This study developed a novel AOC method that combines the standardisation of using known strains of bacteria, with the speed of flow cytometric enumeration, enabling AOC to be routinely and extensively sampled within operational DWDS. The new AOC method was applied to both water treatment works (WTW) and DWDS to successfully validate the method and determine how AOC fluctuates on a spatial and temporal basis. AOC analyses provide first time evidence of pipes and service reservoirs (SR) exhibiting different AOC and (re)growth behaviour, with AOC being found to increase within the majority of pipe only sections of the DWDS, but decrease within SR. To ensure a uniquely holistic view of the impact of AOC within DWDS, both the bulk water and, critically, the attached biofilm phase were studied. Biofilms were developed for 12 months in three purpose-built, full-scale pipe loop test facilities, each supplied by post-treated water containing very different AOC concentrations. Each system replicated the hydraulic retention time, water chemistry and microbiology of operational DWDS, whilst enabling laboratory level control of bulk water and biofilm sampling, thus overcoming limitations of bench scale studies. By following the 12 month growth period with a series of flushing steps, it was possible to assess the mobilisation of material and its correlation to the AOC concentration. AOC concentration was found to impact the cell count, community composition and physical structure of the biofilm during growth, and the amount of material mobilised (and therefore discolouration risk) following flushing. This thesis presents original evidence of AOC cycling within the biofilm, advancing our understanding of how and why AOC concentration varies within DWDS and the impacts this has on microbial (re)growth. A unifying conceptual model is presented that describes the complex AOC processes in DWDS, capturing both bulk water and previously overlooked, biofilm processes. Ultimately, the information gained in this study will enable better management of DWDS environments to maintain the quality of drinking water from source to tap, essential in the future management of biological stability.