Headwater streams are important habitats for aquatic organisms. Within forested regions, headwater streams and riparian corridors function as zones of deposition, storage, processing, and subsequent transport of organic matter. In forested streams, organic matter from the adjacent forest provides the major fuel for the aquatic ecosystem. Along with habitat, headwater streams perform many valuable ecosystem services such as nutrient, hydraulic, and sediment retention; provide thermal refuges; and function as important sites of secondary production for higher animals. Headwater streams are being subjected to many anthropogenic impacts including dams, urbanization, agriculture, forestry, and mining. Ecologists should promote the importance of headwater streams, as well as devote more research to examining entire stream networks, rather than just studying individual longitudinal linkages.