Aphids are arguably the single most damaging group of agricultural insect pests throughout the world. Plant resistance, specifically tolerance, is viewed as an excellent management strategy. However, the use of tolerance for pest management is limited primarily because the mechanisms and the genetics of plant tolerance remain unknown. To uncover these mechanisms, two different barley genotypes known to possess different characteristics on Diuraphis noxia resistance (susceptible and tolerant) were used to profile D. noxia-responsive genes. Plants were infested with 10 D. noxia for 3h, 3d, and 6d. GeneChip Barley Genome arrays from Affymetrix were used to identify transcripts differentially expressed among the two barley genotypes challenged with aphids. A total of 4086 genes were differentially expressed over the three harvest dates in resistant and susceptible barley in response to D. noxia feeding. The greatest number of genes was differentially expressed at 3 d after aphid introduction. A total of 909 genes showed levels of change unique to the resistant barley. These genes were assigned to the following functional categories: abiotic stress, cell maintenance, cell wall fortification, development, direct defense, signal transduction, and unknown function. Several genes from these categories were newly identified to be involved with plant defense responses to aphid feeding. These genes and others may play a critical role in the defense response, and in this case contribute directly to the resistance response, in these barley plants.