In this paper, advantages and limits of the module data base as a tool for engineers respectively specialists in the field of membrane separation processes to select modules in advance of pilot scale and detail engineering are presented. At the beginning, the problem is characterized by two very contrary examples (production of ultra pure water from drinking water and cleaning of waste water from a waste dump). Compared with the well-known tools like vapor-equilibrium data for thermal processes there was a lack of information for membrane separation processes in respect of their use as an unit operation. In the second part, structure and function of the module data base are discussed in detail. The three main parts of the data base, "basic information about the module" with the data given by the manufacturer, "calculation part" to compute the performance of RO-modules under selfdefined conditions, and "extended data" with a large set of information about application-dependent module performance, are presented with some examples. The last part of the paper gives an example to use the module data base for preselection of modules. Again, the limit of such a data base is shown and the difference to a knowledge base system expressed.