Powdery mildew is a common fungal disease of many monocotyledonous and dicotyledonous plant species. In a moderately temperate and humid climate, these ascomycete fungi cause severe yield losses in a wide range of crops. In recent years, several plant genes encoding proteins that control disease resistance against powdery mildew were isolated from the model organism Arabidopsis and the crop barley. Here we review the presumptive biochemical functions of the respective proteins and discuss potential mechanisms which mediate pathogen recognition, resistance signalling, and the termination of host colonization. Perhaps not surprisingly, these advances also promise to shed light on the molecular basis of pathogenesis and biotrophic lifestyle.