Membrane fouling (deposition of water substances on the membrane surface) is a crucial problem during ultrafiltration for drinking water production. Despite many international research efforts the phenomenon of fouling is only partly understood, mainly because of the complexity and variety of influencing factors. These reasons are described in the following article which shows, how mechanistic studies can provide new insight into membrane fouling. Fouling models gained from lab experiments are presented and compared to first results from a pilot plant. The main outcome of the presented study: Interactions between different foulants can impact fouling significantly.