Extraneous water from infiltration/inflow sources reduces the capability of sewer systems and treatment facilities to transport and treat domestic and industrial wastewaters. Infiltration occurs when existing sewer lines undergo material and joint degradation and deterioration as well as when new sewer lines are poorly designed and constructed. The elimination of infiltration/inflow by sewer system rehabilitation can often substantially reduce the cost of wastewater collection and treatment, but a logical and systematic evaluation of the sewer system is necessary to determine its cost-effectiveness. This document is intended to provide engineers, municipalities, and regulatory agencies with guidance on sewer system evaluation.