When using NF/RO membranes in large-scale (drinking) water applications a stable operation of the installation is required to minimize costs. Fouling of membranes will cause an increase in costs due to an increased energy demand, additional labor and costs for cleaning and a shorter lifetime of the membranes. Essential in a proactive operation of an NF/RO plant is an early indication of fouling and a good identification of the type of fouling. For the early indication of fouling normalization of the operational data is required. In addition a number of tools are available to predict the fouling potential of feed water. The combination of the normalization and the proper tools enables the users and operators of NF/RO plants to improve the operation and cleaning efficiency of their plant.