Problems were studied of infiltration of ground and surface waters into sanitary sewers and inflows from various connections to these sewers. Such water enters sewers through joints, cracks, breaks or indirectly through perforated or loose manhole covers or other faulty sewer structures. Inflow water is piped into the sewer from basement and foundation drains, roof leaders, and other legal or illegal connections of storm sewers and combined sewers. Two hundred and twelve public jurisdictions in the United States and Canada were contacted, and twenty-six communities were visited. Practices of consulting engineers and state and provincial water pollution control agencies were also surveyed. Twenty recommendations are given indicating the need for extensive investigation of the extent of the infiltration/inflow problem before relief sewers are constructed or wastewater treatment plants built or enlarged. (WRSIC abstract)