Concerns about waterborne viruses and protozoan cysts developed in the third quarter of the twentieth century, and filtration research for microorganism control in the 1960's through 1980's has reflected these concerns. The chapter briefly reviews historical filtration research for control of bacteria. It provides a comprehensive review of modern research for removal of bacteria, viruses, and protozoan cysts. Processes discussed are slow sand filtration, diatomaceous earth filtration, and the variations of coagulation-filtration (rapid sand filtration, direct filtration, and in-line filtration). Pilot plant data were emphasized, but when possible the results of full scale filter plants are also presented. The inactivation of organisms by disinfectants causes an apparent removal when culture techniques are used to evaluate filtration efficacy, so a careful effort was made in the chapter to present only data that were from studies done prior to disinfection or from research where no disinfection was carried out. Therefore, the results presented should reflect the ability of filters to physically remove the specific microorganisms being discussed.