Filtration is an important treatment process in the removal of pathogenic microorganisms and the prevention of waterborne disease. Historically, filtration was responsible for reducing death and illness from waterborne disease in 1871 in Germany. Other early examples in the U.S. are given which demonstrate the influence of filtration on the decline of morbidity and mortality from typhoid fever. Recent waterborne disease outbreaks are examined and the causes attributed to the lack of filtration or faulty operation and maintenance of the process. Pilot plant data and results from full-scale operation are presented to illustrate the effectiveness of various forms of filtration in reducing microbial populations to concentrations that are easily controlled by disinfection.