The paper reviews principles involved in the processes commonly used to filter drinking water for public water systems. The most common approach is to chemically pretreat water and filter it through a deep (2-1/2 to 3 ft) bed of granular media (coal or sand or combinations of these). The process may involve coagulation and filtration; coagulation, flocculation and filtration; or coagulation, flocculation, sedimentation and filtration. In any case, pretreatment with coagulant chemical is necessary, because particle removal occurs through the mechanism of attachment to grains of filtering material. In contrast, in typical diatomaceous earth filtration practice, coagulant is not used and particle removal by straining is an important aspect of the process. Slow sand filters are used to treat high quality water. No pretreatment coagulants are used, but a biological population develops in the 3 to 4 foot deep sand filter and removal by predation and attachment to sand surfaces and the top slimy layer above the sand (schmutzdecke) cause the quality improvement in the process.