AbstractPlants have evolved systems analogous to animal innate immunity that recognise pathogen‐associated molecular patterns (PAMPs). PAMP detection is an important component of non‐host resistance in plants and serves as an early warning system for the presence of potential pathogens. Binding of a PAMP to the appropriate pattern recognition receptor leads to downstream signalling events and, ultimately, to the induction of basal defence systems. To overcome non‐host resistance, pathogens have evolved effectors that target specific regulatory components of the basal defence system. In turn, this has led to the evolution in plants of cultivar‐specific resistance mediated by R proteins, which guard the targets of effectors against pathogen manipulation; the arms race continues. BioEssays 28: 880–889, 2006. © 2006 Wiley periodicals, Inc.