Polyelectrolytes are macromolecules that contain a large number of charged groups. These charged polymers have found extensive use in a number of water and wastewater treatment processes. The reasons are their high molecular weight and their abillity to attach strongly to charged particle surfaces and biopolymer molecules via electrostatic interactions. A brief description is given of some of these processes, and the way in which polyelectrolytes increase the efficiency of wastewater purification is explained.