One of the most significant questions in plant pathology is: How do pathogens suppress or otherwise manipulate host defences to establish disease? To do this, they secrete proteins and other chemicals that can interact with the host cell. In some cases, virulence determinants called effector proteins are delivered (translocated) inside living host cells where they ‘re-programme’ defences and metabolism to the pathogen’s benefit. My group mainly studies effector proteins from the potato late blight pathogen, Phytophthora infestans. P. infestans is an oomycete, a eukaryotic pathogen with a fungus-like lifestyle that develops haustoria - finger-like cell structures that form an intimate interaction with the host plasma membrane during the early stages of disease. We have shown that haustoria are a major site of delivery of a class of proteins called RXLR effectors. The RXLR motif within these secreted effectors is required for their translocation inside host cells. In the genome of P. infestans we predict that there are >425 RXLR effector genes, demonstrating a remarkable potential for manipulation of host processes. We are discovering that RXLR effectors interact directly with a range of regulatory proteins in the host cell to suppress or otherwise manipulate plant defences. In contrast, the RXLRs are also targets for host resistance proteins which activate immune responses that prevent further colonization by the pathogen.