The co-evolutionary arms race model for plant-pathogen interactions implies that resistance (R) genes are relatively young and monomorphic. However, recent reports show R gene longevity and co-existence of multiple R genes in natural populations. This indicates that R genes are maintained by balancing selection, which occurs when loss of the matching avirulence (Avr) gene in the pathogen is associated with reduced virulence. We reason that balancing selection favors R proteins that function as guards, monitoring changes in the virulence target mediated by the Avr factor, rather than recognizing the Avr factor itself. Indeed, the available experimental data support the notion that guarding is prevalent in gene-for-gene interactions.