Municipal wastewater is the general term applied to the liquid wastes collected from residential, commercial, and industrial areas of a city. In addition to these waste streams, some undesired groundwater and surface runoff may also enter the collection system. All these flows are conveyed by means of a sewerage system to a central location for treatment. There is a wide variation in seasonal, daily, and hourly flow rates of the municipal wastewater, and several flow terms are used to identify these flow variations and characterize the municipal wastewaters. The purpose of this chapter is to present the flow characteristics of municipal wastewater.