Pilot-scale distribution facilities are used to estimate the influence of water quality parameters on full-scale distribution system performance. Pilot facilities are used when access to the full-scale system is limited, when the parameters to be tested may significantly alter the quality of water delivered to the consumer, or if more tightly controlled conditions are required. Since pilot system results are extrapolated to the distribution system, it is critical that the reactors be designed to simulate relevant operational conditions including shear stress, pipe materials, temperature, and disinfectant types and concentrations. This paper discusses the selection of a model distribution system and the advantages and disadvantages of pipe loop systems and annular reactors as pilot-scale systems.