An important challenge for nanofiltration processes is the development of predictive models that convey a fundamental understanding and simple quantification of the governing phenomena in a way that has the potential for industrial application. The paper reviews one such approach, including: the formulation of a mathematically consistent description of rejection and flux at nanofiltration pores, the inclusion in calculations of pore size distributions with pore size dependent physical properties, the linearisation of these models to facilitate rapid calculations, and the use of such calculations to specify membranes for technically demanding separations. The present achievements and future challenges for predictive modelling of nanofiltration processes are assessed.