Mistletoes are familiar to most Europeans and North Americans because of the Christmas folklore associated with these parasitic flowering plants (33,116). Some may also know these plants are parasites of trees but do not realize that mistletoes are widespread, ecologically important components of forests worldwide. Although some mistletoe species are damaging pathogens, most do not impact economically valuable crops and forest products but actually play key roles in forest ecosystems. Particularly in Loranthaceae, coevolutionary relationships with birds (involving pollination and seed dispersal) have fueled several adaptive radiations, thus producing one of the most diverse and fascinating life forms on our planet. Here we summarize mistletoe biology, pathology, and management as well as current ecological concepts and their evolution as revealed by molecular phylogenetics.