By reducing the levels of biodegradable organic matter available to react with disinfectants, biological drinking water treatment has considerable promise in helping utilities to meet stricter byproduct regulations without compromising disinfection. A preliminary framework is emerging, based on both mechanistic and empirical models, which will allow feasibility assessments and preliminary design of biological treatment processes for a given water. Currently, such model predictions must be confirmed by pilot studies. The lack of finished water criteria for BOM, chlorination byproduct precursors and chlorine demand hampers the rational design of biological treatment processes. Such criteria must await the establishment of definitive cause-and-effect relationships between various water quality parameters and bacterial regrowth, byproduct formation and disinfectant residual depletion in distribution systems. Design and operating criteria relating to the particle removal aspects of biological filters are urgently required. Operational strategies are needed to ensure that elevated levels of bacterial endotoxin or GAC fines are not present in biologically treated water discharged to the distribution system.