Controlling fouling of membrane filtration installations (nanofiltration and reverse osmosis) is a major challenge for the water industry. Fouling affects the plant performance and increases the costs of plant operation. Diagnosis of the type(s) and extent of fouling is the essential first step for controlling fouling in order to maintain plant performance. A quick scan of the installation for the diagnosis of fouling involves an on-site study of a membrane element (autopsy), including evaluation of data of pretreatment. Comparison of the results of the quick scan with the database with results of autopsies related to plant performance (NPD, MTC), chemicals dosed (e.g. scale inhibitor) would show whether fouling is present and elucidates the types and extent of fouling present. The database also enables the evaluation of the risk on biofouling. Diagnosis of the type of fouling enables directed and more effective actions for prevention and control of (bio)fouling. Studies at (pilot) plants showed that operational problems can be caused by other types of fouling than expected for and that dosed chemicals can pose a risk for (bio)fouling. These case studies emphasize the significance of diagnosis of the type of fouling. Also, tools for prevention and prediction and studying the effect of cleaning strategies on fouling are presented.