Reverse osmosis (RO) treatment systems have features which make them attractive for bacteria to settle, colonize and form biofilms. Often the water is warm, at least seasonally, and provides a continuous source of dissolved, assimilable nutrients; the combination of which provide high growth conditions for bacteria. Also, the RO element itself has a high proportion of surface area for the bacteria to attach. Interestingly, the entire RO system does not equally become inhabited by bacteria. It is found that the feed side of the system is more prone to colonization and biofilm formation than the reject side of the system. Comparing the environment (e.g., velocity, flux, solute concentration) of each side of the RO system provide some parameters to explore in an effort to identify mechanisms for reducing biofilm formation in RO systems. Fundamental research highlighting some of the more influential parameters will be presented.