The introduction of exotic tree pathogens into new regions often results in epidemics, mortality of trees and other plants in forested areas and landscape plantings. Several ecologically significant and economically important tree diseases have been introduced into North America, including Chestnut Blight of Fagaceae, White Pine Blister Rust of Pinaceae, and Scleroderris Canker of various conifers. These introductions resulted in ecological degradation due to the loss of principal (climax) tree species and wildlife habitat followed by increased fire hazard and soil erosion. Similar scenarios resulted from the introduction of exotic tree diseases into Europe including Canker Stain of Platanaceae, Seridum Canker of Cupressaceae, Alder Decline of Betulaceae, Phytophthora kernoviae-caused Bleeding Canker of Ericaceae and Fagaceae, as well as Dutch Elm Disease of Ulmaceae. Phytophthora ramorum Werres, De Cock and Man In’t Veld is a recently described organism of unknown origin. Infection causes leaf spots, cankers and death of native plants and landscape ornamentals. Several diseases caused by P. ramorum have been described including Sudden Oak Death, Ramorum Leaf Blight and Ramorum Dieback. The organism currently challenges the plant regulatory community due to several factors. Phytophthora ramorum has a very large host range and is associated with numerous “pathways” of natural and human-assisted movement. Multiple pathways have resulted in the regulation of numerous plant products from various industries. In addition, infection by the pathogen can result in disease latency. Several lineages and mating types provide the potential for sexual recombination. Because the pathogen can be difficult to isolate from specimens collected from the field it can be problematic to identify to the species level without molecular diagnostic methods. The discovery, identification and additional study of P. ramorum raise the awareness of scientists and regulators to the existence of several other undescribed or incompletely known species of Phytophthora, which also have the capacity to move regionally and internationally. A synopsis of the history of the disease, causal organism, and regulations enacted to limit the spread of P. ramorum in North America and Europe are provided.