Plant disease resistance genes trigger defence mechanisms upon recognition of pathogen compatibility factors, which are encoded by avirulence genes. Isolation of the barley powdery mildew resistance gene Mla opens the door to understanding the extensive allelic diversity of this locus. Completion of the Arabidopsis genome sequence enables the analysis of the complete set of R-gene homologues in a flowering plant. A new R gene, RPW8, conferring resistance in Arabidopsis to powdery mildew, reveals a new class of protein associated with pathogen recognition. New prospects for using R-gene polymorphism in agriculture are becoming apparent.