Several recent studies have quantified the performance characteristics of ultrafiltration membranes in terms of the inherent trade-off between the membrane selectivity and permeability. However, none of these studies have accounted for the effects of membrane fouling on the evolution of the selectivity and permeability during typical ultrafiltration processes. This review paper examines a range of available fouling models, including the classical pore blockage/pore constriction models as well as newer models based on depth filtration and solute adsorption, with a particular focus on understanding the effects of these fouling phenomena on the permeability-selectivity tradeoff. Although fouling always causes a reduction in permeability, the selectivity can actually increase, e.g., if the larger (less selective) pores are preferentially blocked during ultrafiltration or if the pores are constricted by the foulants. The evolution of the permeability-selectivity tradeoff can be quite complex, depending on both the underlying fouling mechanism as well as the distribution of pore and solute sizes. These results provide new insights into the behavior of ultrafiltration processes. (C) 2013 Elsevier B.V. All rights reserved.