Granular media filtration as applied to water and wastewater treatment is reviewed in terms of the types of filter media available, the alternative filter configurations, their modes of operation, and backwashing procedures. Filtration kinetics and mechanisms are reviewed, together with the hydraulic equations which have been developed to predict the pressure drop across a filter bed. Studies on the physical parameters which affect filter performance are reviewed. These include flow rate, particle size, filter media grain size, and particle number concentration. The effects of variation of chemical parameters, such as pH, ionic strength, and ionic species, on the surface chemistry of filter systems and filtration efficiency are reviewed. The use of polyelectrolytes in filtration, and possible mechanisms for their role as filtration aids are discussed. Studies on the influence of polymer structure, and the time and degree of mixing of the polymer with wastewater, prior to filtration, are reviewed. Methods of screening polymers and optimizing the dose for filtration applications are reviewed, together with other techniques which have been used in mechanistic studies of the process.