Stripe rust (also called yellow rust, Yr) of wheat is caused by Puccinia striiformis f. sp. tritici (Pst), and stripe rust of barley is caused by Puccinia striiformis f. sp. hordei (Psh). These fungal pathogens continually produce new virulent races that may circumvent race-specific resistance genes in commercially grown cultivars of the crops. To effectively develop new resistant cultivars and manage diseases, it is essential to monitor virulence changes and characterize the races of the pathogen. This chapter describes the protocols used to identify races of the pathogens in the United States. Currently, a set of 18 Yr single-gene lines is used to differentiate races of Pst, and a set of 12 barley genotypes is used to differentiate races of Psh. As both Pst and Psh can infect other cereal crops and grasses, some Pst races can infect few barley varieties, and some Psh races may infect few wheat varieties, a scheme we have been using to address the across virulence issue is presented and discussed.