Ultrapure water (UPW) can be defined as water which has been highly purified to extremely precise specifications including the removal or organic particles and dissolved gases. Three stages make up the UPW system including a pretreatment stage to provide the purified water, a primary stage to purify the water further and a polishing stage. This article evaluates the different UPW end-users such as power generation, pharmaceuticals, microelectronics and specialised applications and discusses common treatment technologies in each of the industries associated with UPW.