More than 1900 species of black flies have been described from Planet Earth. Of these, less than 10% have any socioeconomic importance. The perspective of black flies as pests and vectors of disease agents has been balanced in recent years with a greater understanding of their driving role in aquatic and terrestrial ecosystems. Black flies serve as food not only for invertebrate and vertebrate predators, but also for humans and plants. In many aquatic ecosystems, they play a keystone role as the major processors of fine particulate organic matter. Viewed in this new light, black flies now must be managed not only as pests, but also as keystone organisms in flowing waters and as vital links to terrestrial processes.