The 1984 report of A2 mating types of Phytophthora infestans (Mont.) de Bary in western Europe (20) was the first indication of new and dramatic developments in populations ofthat Fungus. This discovery stimuiated plant pathologists aH over the world to analyze local populations, since previously only the A 1 mating type had been detected outside of central Mexico (Fig. I). The analyses of a large number of dispersed Eocal populations indicated, surprisingly, that the changes were not restricted to western Europe but, rather, were worldwide (Fig. 2) [3,10,23,26,36,41). The recent wortdwide changes in populations mOSK certainly result from migration. Indeed, migration has played an essential role in the entire history of potato late blight. In this article we illustrate that role. To provide context, however, we first present background concerning the basic biology/pathology of P. infestans, the genetic tools used to investigate populations of P. infesrons, and the char. acteristics of the source population of P. infestans.