This paper describes biofouling as a biofilm problem, and how it is important to understand basic biofilm processes and properties in order to design rational countermeasures. It also describes appropriate countermeasures and prevention options available to membrane plant designers and operators to combat biofouling such as: scaling control which is typically required for all reverse osmosis/nanofiltration (RO/NF) membrane systems in either surface or groundwaters and is achieved by acid and/or antiscalent addition; plugging control which is typically required for all RO/NF membrane systems in either surface or groundwaters and is achieved by feed water turbidities and SDI's less than 0.2 NTU and 2, respectively; bio-fouling control which is typically required for aerobic surface or groundwaters and is achieved by NH₂Cl or addition of other bactericidal agents; and, organic fouling which can occur in surface water systems with TOC > 3-6 mg/L and is typically reduced by coagulation, sedimentation and filtration. Includes tables, figure.