The invasive sudden oak death pathogen Phytophthora ramorum emerged repeatedly since its first detection in the 1990s in the U.S.A. and Europe. This paper will explore recent research by several groups documenting the patterns observed and mechanisms inferred to explain these patterns. Briefly, three distinct clonal lineages are recognized named NA1, NA2, and EU1 named consecutively after the continent of origin on which they where first found, namely North America (NA) and Europe (EU). While all three clonal lineages are found in Canada and the U.S.A., Europe to date only has the EU1 clonal lineage. Detailed phylogeographic analysis has documented that the introduction of the NA1, NA2, and EU1 clonal lineages originated from separate populations. The NA1 lineage was introduced into California from an unknown source population, while the NA2 and EU1 lineages were introduced into the Pacific Northwest (either British Columbia or Washington). While the source population for the NA2 introduction remains to be established, coalescent analysis supported introduction of the EU1 lineage into North America from Europe. Further emergence of P. ramorum lineages is likely, given the observed repeated emergence of P. ramorum over the last two decades and appears to be driven by shipment of infested nursery material.