Today's high technology water purification systems are designed to remove many types of impurities. One of the most difficult to deal with is microbiological contamination. Microbial growth in a water system presents two problems: it not only reduces water quality, but if left unattended, the problem can 'grow' to reduce system performance and life. Biofouling can cause a decrease in product water flow rate, a decrease in driving pressure, and can contribute to corrosion of piping and housings. This paper is a primer on the topic of dealing with established and potential microbial problems in crossflow membrane systems.