To provide a better and more fundamental understanding of natural organic matter (NOM) fouling of membranes, a comprehensive study, funded by AwwaRF, was conducted that focused on characterization of fouling of microfiltration (MF) and ultrafiltration (UF) membranes by three classes or categories of NOM foulants: allochthonous, represented by humic and fulvic acids; autochthonous or algal-derived; and, effluent organic matter. Study objectives included identifying problematical NOM foulants, quantifying the NOM fouling potential of a variety of waters, distinguishing between NOM fractions that contribute to reversible and irreversible fouling, assessing the properties of membranes that influence fouling and developing a predictive tool or surrogate test that could be used to measure the "NOM fouling index" of a given water. The research was conducted by using a combination of bench, pilot and full-scale tests conducted with a variety of flat-sheet and hollow-fiber MF/UF membranes. Stirred cell and small-scale hollow fiber bench test apparatuses were evaluated for fouling prediction and results compared with pilot to assess viability. A variety of analytical methods, included LC/DOC, XAD resin, AFM, GC/MS pyrolysis and AFM were employed for foulant characterization in aqueous and solid phases. Bench results show clear differences in fouling potential by different waters/NOM types, for different membrane materials on a given water and between water and chemical backwashes. Good correlations were observed between different bench test methods. Characterization of NOM from raw, filtrate and backwash samples show that polysaccharides and large MW fraction of NOM is primary fouling constituent, relatively independent of source or NOM type. The results of this research will provide the drinking water/reuse communities with better understanding of NOM fouling, effectiveness of membrane materials/properties and backwashing/chemical washing in managing NOM fouling and value of rapid, bench scale tests in rapid NOM fouling prediction. This information can be utilized for more cost effective design and operation of full-scale MF/UF facilities. Includes reference, tables, figures.