Introduction Stripe rust, caused by Puccinia striiformis West., is the most important disease of wheat in western North America, and it has become more important in the South-Central United States since 1984. The disease was first recognized in North America in 1915 by F. Kolpin Ravn, a visiting scientist from Denmark, who was traveling in the West- ern United States with a U.S. Department of Agriculture crop survey team (Carleton 1915). Once recognized, the disease was detected throughout the Western United States in 1915 and in Canada within a few years (Humphrey et al. 1924, Johnson and Newton 1928). Later, H.B. Humphrey (Humphrey et al. 1924), after examining herbarium specimens, determined that P. striiformis had been present in western Washington for at least 23 years before it was recognized. Early plant pathologists were aware of the destructiveness of stripe rust in Europe and Asia and were concerned about its possible spread to the major wheat-growing regions in the United States and central Canada (Hungerford 1923, Humphrey et al. 1924, Newton and Johnson 1936). Consequently, several major studies were conducted during the 1920's. However, the only reports on the pathogenicity of P. striiformis in North America were on its host range and on the resistance of local cultivars. Hungerford and Owens (1923) reported that Bromus sterilis was resistant to stripe rust collections from Hordeum jubatum but not to those from Elymus glaucs and Bromus marginatus. That report indicated the existence of races, or strains, of P. striiformis. Newton et al. in 1932 (Newton et al. 1933) and Bever in 1933 (Bever 1934) each isolated two physiologic forms (races) of P. striiformis using differential wheat cultivars. Newton and Johnson (Newton and Johnson 1936) reported that their two races were similar to the race from Montana and the race from Idaho reported by Bever. The race from Montana was virulent on Chinese 166 and Red Russian and avirulent on Heines Kolben. The collection from Moscow, Idaho, was virulent on Heines Kolben and avirulent on the other two cultivars. No reports were published on the virulence of P. striiformis in North America for the next 25 years. Concern about stripe rust diminished, the disease was no longer considered important in North America, and research projects on stripe rust were eliminated. But the situation changed drastically in the late 1950's and early 1960's, when severe epidemics caused massive losses in California and the Pacific Northwest (Shaner and Powelson 1971, Tollenaar and Houston 1967). These devastating effects of stripe rust led to an increased emphasis on breeding for resistance to the rust. Purdy and Allan (Purdy and Allan 1963) observed that at least three stripe rust races existed in Washington. Additional races were subsequently identified in the Western United States (Purdy and Allan 1966; Beaver and Powelson 1969; Line 1972, 1976, 1980, 1984; Volin and Sharp 1973). The races detected in the early 1930's were identified on a set of differential cultivars that had been used in Europe (Gassner and Straib 1932). But that set did not effectively differentiate the predominant races of P. striiformis in North America. Therefore, a set of differential cultivars for the United States was selected in 1969, and a uniform system of describing and naming the races was agreed on by those working on stripe rust in the United States (Line et al. 1970). Since then, the system, with some modification, has been used to monitor the virulence and distribution of pathotypes (races) in North America. Additional differential cultivars were added as new races appeared (Line 1972, 1976, 1980). This report summarizes the results of more than 20 years of research on the differentiation, virulence, and aggressiveness of races of P. striiformis in North America; detection of the races; geographic distribution of the races; interrelationships of the races; and movement of inoculum from region to region.