Membranes have historically been used to remove salts and other inorganic compounds from water but recently both bench-scale and field studies have shown their effectiveness for removing organic compounds from drinking water. High pressure membranes are those using pressures between 150 to 400 psig. These membranes are commonly called reverse osmosis membranes. During bench-scale studies, reverse osmosis membranes tested included cellulose acetate, polyamide, and thin-film composites. At a research site in Suffolk County, New York, removal of agricultural contaminants by reverse osmosis was evaluated on the bench and in a pilot plant. Percent removals for long term pilot plant evaluation for aldicarb sulfone, aldicarb sulfoxide, 1,2-dichloropropane, and carbofuran ranged from 53% to more than 95%. Low pressure membranes are usually operated at or below 150 psig. These membranes, normally called ultrafiltration membranes, were evaluated at various sites in Florida to investigate their efficiency for removing disinfection byproduct precursors. With a system recovery (permeate flow/raw water flow) of 75 percent at one groundwater site, the average reduction of trihalomethane formation potential and total organic halide was 95 percent and 96 percent from raw water averages of 456 micrograms/L and 977 micrograms/L, respectively.