Drinking water contains chemical, microbiological, and other contaminants. A healthy adult drinks about 10-12 liters of water per week, this water goes across a selective barrier of the gastrointestinal tract, and excess chemicals are removed by the healthy kidney. In contrast, with a typical three times a week hemodialysis protocol, a dialysis patient is exposed to more than 300 liters of water weekly, the water passes through the nonselective dialyzer membrane, and there is no kidney to maintain the normal balance of chemicals. Moreover, the highly permeable high-flux membrane used today increases the risk of increased load of contaminants passing through the membrane and into the blood. Some common contaminants have been shown to be injurious to patients. Thus, the water for dialysis must be purified of these contaminants prior to its use by the proportioning system of the dialysis machine to make the final dialysate. This chapter covers the purpose of water purification before it is used for dialysis. It describes the components of a water treatment system, how the system is monitored, and the common contaminants found in water.