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In the plant immune system, according to the ‘gene-for-gene’ model, a resistance <i>(R)</i> gene product in the plant specifically surveils a corresponding effector protein functioning as an avirulence (<i>Avr</i>) gene product. This system differs from other plant–pathogen interaction systems, in which plant <i>R</i> genes recognize a single type of gene or gene family because almost all virus genes with distinct structures and functions can also interact with <i>R</i> genes as <i>Avr</i> determinants. Thus, research conducted on viral Avr-R systems can provide a novel understanding of <i>Avr</i> and <i>R</i> gene product interactions and identify mechanisms that enable rapid co-evolution of plants and phytopathogens. In this review, we intend to provide a brief overview of virus-encoded proteins and their roles in triggering plant resistance, and we also summarize current progress in understanding plant resistance against virus <i>Avr</i> genes. Moreover, we present applications of <i>Avr</i> gene-mediated phenotyping in <i>R</i> gene identification and screening of segregating populations during breeding processes.