This chapter focuses on the use of membrane ultrafiltration (UF) and microfiltration (MF) in the treatment and purification of water. It discusses two very different applications of MF/UF: production of ultrapure water for use in the semiconductor and pharmaceutical industry, and treatment of oily wastewater effluents generated in the textile industry. Ultrapure water is a critical process fluid in both the semiconductor and pharmaceutical industries. Silicon wafers are washed and rinsed in ultrapure water numerous times during the production of complex integrated circuits. MF and UF are both used for the production of ultrapure water, and there has been considerable "competition" between these two membrane processes. Oily wastewaters constitute a major environmental problem in many industries. Metal-working operations use water to remove dirt and grease prior to painting. Hollow fiber, spiral-wound, tubular, and flat-plate modules could all be used for the treatment of oily wastewaters.