Membrane fouling by humic substances (HS) is a major problem in UF water treatment. The complex phenomena involved have been the subject of numerous investigations. HS fouling is attributed to accumulation of particles in the feed water on the membrane surface forming a cake layer and adsorption of small particles in the inner pores thus constricting or blocking the pore mouth. The exact contribution of each of these mechanisms to membrane fouling and to flux reduction is not sufficiently clear. While there is a substantial qualitative agreement on various fouling aspects in studies focusing on commercial humic acid solutions, the extensive literature on fouling by natural organic matter (NOM) is less coherent. The major deficiency in the available information is the lack of reliable predictive models. This paper presents a systematic review of scattered literate information and outlines HS fouling aspects that await elucidation. (c) 2010 Elsevier B.V. All rights reserved.