Plant pathogens establish infection by secretion of effector proteins that may be delivered inside host cells to manipulate innate immunity. It is increasingly apparent that the ubiquitin proteasome system (UPS) contributes significantly to the regulation of plant defences and, as such, is a target for pathogen effectors. Bacterial effectors delivered by the type III and IV secretion systems have been shown to interact with components of the host UPS. Some of these effectors possess functional domains that are conserved in UPS enzymes, whilst others contain novel domains with ubiquitination activities. Relatively little is known about effector activities in eukaryotic microbial plant pathogens. Nevertheless, effectors from oomycetes that contain an RXLR motif for translocation to the inside of plant cells have been shown to suppress host defences. Annotation of the genome of one such oomycete, the potato late blight pathogen Phytophthora infestans, and protein-protein interaction assays to discover host proteins targeted by the RXLR effector AVR3a, have revealed that this eukaryotic plant pathogen also has the potential to manipulate host plant UPS functions.