This work demonstrates that interactions between membranes and natural organic matter (NOM) include rejection by both steric and electrostatic exclusion, and fouling by adsorption. NOM rejection and fouling are influenced by both NOM characteristics (molecular weight, aromaticity/humic content, and charge) and membrane properties (molecular weight cutoff (MWCO), hydrophobicity, and charge) as well as hydrodynamic operating conditions represented by a f/k ratio (the ratio of permeate flux (f) to a diffusional back-transport mass transfer coefficient (k)).