This Practice Note outlines the functions and duties of sewerage undertakers under the Water Industry Act 1991 (WIA 1991). It covers who sewerage undertakers are, their functions and duties, changes under the Water Act 2014, public sewers and lateral drains, the transfer and adoption of private sewers and drains, standards of performance and sewer maps. It looks at requirements under the Urban Waster Water Treatment Regulations 1994, and finally policy and guidance for storm overflows and how to implement measures identified in the Storm Overflows Discharge Reduction Plan, the need for which was introduced by the Environment Act 2021. It was produced in partnership with John Bates of Old Square Chambers.