An impressive array of animals function as ecosystem engineers in streams through a variety of activities, ranging from nest digging by anadromous salmon to benthic foraging by South American fishes, from the burrowing of aquatic insects to the trampling of hippos. These ecosystem engineers have local impacts on benthic habitat and also strongly affect downstream fluxes of nutrients and other resources. The impacts of ecosystem engineers are most likely some function of their behavior, size, and population density, modulated by the abiotic conditions of the stream. In streams, subsidies often control the body size and density of ecosystem engineers, while hydrologic energy controls their distribution, density, and life-history attributes, the habitats they create, and the resources and organisms they affect. Because ecosystem engineers can profoundly affect stream ecosystems, and because they themselves can be significantly affected positively or negatively by human activities, understanding ecosystem engineering in streams is increasingly important for the management of these ecosystems.