The barley stem rust resistance gene Rpg1 was cloned and characterized by a map-based approach. It was shown to have a novel R-gene structure containing tandem kinase domains. To determine the origin of Rpg1 we analyzed alleles from several barley land races (Hordeum vulgare) and wild barley (H. vulgare subsp. spontaneum) accessions. Our allele analysis determined that the functional Rpg1 gene is rare among the landraces and was absent from the wild accessions examined. During the 1990s two new races of Puccinia graminis f. sp. tritici (Pgt) designated QCCJ and TTKSK, virulent on Rpg1, emerged in North America and Eastern Africa, respectively. Analysis of barley accessions for QCCJ and TTKSK reaction identified the barley line Q21861 as the best resistance source and was designated the rpg4 gene. Q21861 also contains resistance against Puccinia graminis f. sp. secalis (Pgs) designated the Rpg5 gene. The two resistance genes initially co-segragated to a region on barley chromosome 5H(7). In an attempt to quickly identify an rpg4/Rpg5 candidate gene, five Rpg1 gene family members were identified and mapped. One was tightly linked to the rpg4/Rpg5 locus but segregated away in a high-resolution mapping population. Recombinant analysis of the high-resolution population revealed that rpg4 and Rpg5 were distinct genes. We cloned and partially characterized a 70 kbp genomic region from barley containing the rpg4 and Rpg5 genes. An Rpg5 candidate gene was identified and confirmed by allele sequencing. The predicted RPG5 protein from the resistant line Q21861 contains a Nucleotide-Binding-Site, Leucine-Rich-Repeat region and a protein kinase domain. A reverse genetics approach, Virus Induced Gene Silencing was used to confirm the Rpg5 gene. A gene encoding an actin depolymerizing factor-like protein (ADF2) was identified as rpg4. The Adf2 gene (rpg4), in combination with Rpg5, appears to be essential for resistance against the Pgt races, but not for the Pgs isolate. One possible hypothesis for the Adf2 gene function is that it might be activated or inactivated by RPG5 upon pathogen recognition. Thus making the ADF2 protein inaccessible for the fungus to redirect actin networks to establish a haustoria-plant interface to obtain nutrients from the plant.