Book excerpt: Hemlock dwarf mistletoe (HDM) (¿Arceuthobium tsugense¿) is a small, inconspicuous parasite that has significant effects on tree growth and stand structure in coastal forest ecosystems of west. N. Amer. Previous clearcut harvesting of areas that removed all infected trees, and forestry practices that established even-aged stands of trees effectively prevented HDM impacts. However, current forest practices that restrict clearcut harvesting to small openings, and retain live trees to preserve attributes of old-growth forests create conditions that are favorable for enhanced seed production by HDM, early spread of the HDM to infect young trees, and, increased growth impacts to residual trees. More info. is needed on the biology and impacts of HDM. Illustrations.