Natural organic matter (NOM) is ubiquitous in the aquatic environment and its presence is known to lead to fouling of low-pressure microfiltration (MF) and ultrafiltration (UF) membranes. The extent of membrane fouling based on water quality measurements is difficult to estimate, and much research has taken place to understand the phenomenon that adds considerably to the operating costs and productivity of membrane systems. Early NOM fouling experiments concentrated on fouling via hydrophobic NOM components, with fouling occurring from either direct adsorption of these compounds or via Ca2+ bridging. These results were mainly obtained from experiments using purified humic acid solutions. Later experiments that have used natural surface waters have demonstrated the large fouling potential of high molecular weight hydrophilic NOM. It is now emerging that interactions between organic components may be important, as well as interactions between the organics and the membrane, and results demonstrating this are presented. Includes 13 references, tables, figures.