In the production of ultrapure water for the power and microelectronics industries, multiple pass reverse osmosis (RO) process is commonly the major step in the reduction of dissolved and suspended matter before polishing by ion exchange and other methods to attain the high purity requirements. With the diverse location of power plants and microelectronic manufacturing facilities around the world, silica concentrations in source waters can range between I and 60 ppm (mg/L) to even 300 ppm in some volcanic regions. High pressure steam generators and fine microelectronic structures now require water containing less than I ppb (ug/L) concentrations of silica. In designing purification processes, silica has presented issues not only as formidable challenges in many locations as RO membrane foulants, as well as a contaminant requiring efficient removal. Analyses of RO membrane foulants and correlation with water chemistry in the course of trouble-shooting numerous RO processes continues to offer us opportunities to understand silica chemistry, the patterns of silica fouling and methods by which we can chemically control the RO process. Such understanding is applicable to the operation of ion-exchange resin beds as well. In this paper we review the speciation of silica in feedwaters, and chemical approaches to controlling fouling and maximizing silica reduction. Silica and silicates are addressed in the three categories of reactive soluble, non-reactive soluble (colloidal, not filterable) and non-reactive insoluble (particulate, filterable) forms. A brief review of geochemistry, the chemical and biochemical dissolution and deposition of silica and silicates in nature is provided for insights and understanding of natural processes that can be applied to the task of process design and control in silica removal from water.