Transparent exopolymer particles (TEP) as gel-like particulate acidic polysaccharide have been commonly found in marine, surface water and wastewater. Currently, increasing interest has been devoted to TEP-associated membrane fouling in different membrane systems for water and wastewater treatment, thus this review attempts to provide a holistic view and critical analysis with regard to the definition, formation, detection and properties of TEP, which could ultimately determine its fouling potential. It appears that there is not a common consensus on the actual role of TEP in membrane fouling development due to the subjective definition and highly debatable detection method of TEP. It was clearly demonstrated in this review that the formation of TEP was largely related to cations in water and wastewater which indeed determined the cross-linking degree of precursor materials (e.g. polysaccharides) via intermolecular interactions, and subsequently the quantity of TEP formed. The binding between cations ions (e.g. monovalent, divalent and trivalent cations) and polysaccharide not only depends on the functional groups of polysaccharide, but also its spatial configuration. These in turn suggest that the formation, property and ultimate fouling potential of TEP would be closely related to the type and concentration of cations, while well explaining the controversial reports on TEP-associated fouling in the literature. In addition, the fouling mechanisms of TEP are also elucidated with details in this review, including (i) the formation of TEP-associated gel layer on membrane surface; (ii) carrying microorganisms to membrane surface via protobiofilm and (iii) trapping of deformable TEP in membrane pores. Consequently, it is apparent that TEP is an ignored determinant of membrane fouling, which has not yet been seriously addressed in the design and operation of membrane systems for water and wastewater treatment.