This presentation explains many of the genetic characteristics (general and unique) of Phytophthora infestans. It starts with a chronological description of the gradually increasingly accurate understanding of the genetics of this oomycete. It emphasizes the population genetics worldwide, but with special emphasis on the United States. A major theme is that "migration" has played a huge role in the diversity of populations in the USA and worldwide. An accurate understanding of the simple population genetic structure of this organism in the USA can be used to improve efforts to mitigate the harmful effects of this pathogen.