Plant disease resistance (R) proteins confer protection against specific pathogens or pathogen isolates. R proteins function by recognizing pathogen-encoded avirulence (Avr) proteins and translating this recognition event into an initiation of downstream signaling pathways. Key to understanding this process is the study of the protein–protein interactions involving R proteins. Recognition and signaling mechanisms are mediated by both intramolecular interactions that take place between different domains of R proteins as well as intermolecular interactions between R proteins and additional plant proteins. These processes have been studied in part by using Agrobacterium -mediated transient expression of R protein fragments in Nicotiana benthamiana which allows for the rapid assessment of functionality. Furthermore, pairs of proteins or protein fragments can be transiently expressed as fusions with different epitope tags. One putative protein partner is subjected to immunoprecipitation. Subsequent immunoblotting is performed to determine whether the second protein has remained associated (or co-immunoprecipitated) with the first, indicating a protein–protein interaction. This technique has contributed substantially to structure–function analyses of R proteins and to the characterization of interactions between R proteins and other plant proteins.