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Rainfall dependent inflow and infiltration (RDm into sanitary sewer systems has long been recognized as a source of operating problems in wastewater collection and treatment systems. RDll can cause sanitary sewer flows to increase during wet weather to rates that exceed the hydraulic capacity of the wastewater collection system in one or more locations. When this occurs, the hydraulic grade line is elevated to a level that can cause sanitary sewer overflows (SSOs), with the resulting surcharged flows entering basements, overflowing to the street surface through manholes, or discharging to nearby streams through constructed overflow outfalls. RDII also contnbutes to serious operating problems at wastewater treatment facilities, including hydraulic overloading and disruption of biological and other plant processes. Thus, the need to understand and control the RDll problem is crucial. Two very different hydrologic processes are involved in the sanitary and combined sewer systems in response to wet weather. The more straightforward and well-understood process is that of the combined sewer system, where surface runoff is the dominant response. However, hydrologic processes in the sanitary sewer system are not as well understood, nor are they as accurately modeled with reasonably available data. As a result, empirical data are used to estimate the hydrologic response in the sanitary sewer system, rather than detenninistically model the physical process. Attempts to simulate the sanitarysewerresponseusing algorithms and tools developed for simulating combined sewer response typically yield less reliable results and solutions.