More than 50 years ago, Flor (1) proposed a model to describe plant–pathogen interactions based on genetic studies with flax and the flax-rust pathogen. His “gene-for-gene” model predicted that plant resistance would occur only when a plant possesses a dominant resistance gene (R) and the pathogen expresses the complementary dominant avirulence gene (Avr), conferring strain specificity. An alteration or loss of the plant resistance gene or the pathogen Avr determinant leads to disease in the host. The R gene products are hypothesized to act as receptors for the products of the Avr locus. As a result of intense research in the last 10 years, it is now well established that Flor’s model still holds true for many host–pathogen interactions.