The powdery mildew fungi have proven to be a useful model system for studies of the host-parasite interface in biotrophic parasitism. Investigation of the interface has requrred the development of novel experimental approaches, for example the isolation of populations of haustoria in association with other interface components and the chemical and physical manipulation of living isolated epidermal strips infected wth powdery mildew fungi. These experimental approaches have provided information on the nature of metabolites transferred from host to pathogen at the interface and on the underlymg mechanisms. Studies of incompatible interactions with powdery mildew fungi have indicated that the establishment of a functional host-haustorial interface is critical for successful infection. In future, the application of recombinant DNA and monoclonal antibody technologies to the host-haustorium interface of powdery mildews should lead to a more detailed molecular analysis of the interface, and thus provide new insights into its structure and function.