In spite of all promising perspectives for nanofiltration, not only in drinking water production but also in wastewater treatment, the food industry, the chemical and pharmaceutical industry, and many other industries, there are still some unresolved problems that slow down large-scale applications. This paper identifies six challenges for nanofiltration where solutions are still scarce: (1) avoiding membrane fouling, and possibilities to remediate, (2) improving the separation between solutes that can be achieved, (3) further treatment of concentrates, (4) chemical resistance and limited lifetime of membranes, (5) insufficient rejection of pollutants in water treatment, and (6) the need for modelling and simulation tools. The implementation of nanofiltration in the industry is a success story because these challenges can be dealt with for many applications, whereas more research would result in many more possible applications. It is suggested that these challenges should be among the main priorities on the research agenda for nanofiltration. This paper offers an overview of the state-of-the-art in these areas, without going into details about specific observations in individual studies, but rather aiming at giving the overall picture of possible drawbacks. This leads to suggestions which direction the nanofiltration research community should follow, and where research questions can be found. Evidently, the six identified challenges are to some extent interrelated; mutual influences are explained as well as possible solutions, or possible pathways to solutions. (C) 2008 Elsevier B.V. All rights reserved.