The benthic macroinvertebrates of running waters include many orders of organisms that represent an interesting, usually unappreciated part of the aquatic environment. Benthic fauna in standing waters, which will be briefly discussed in this chapter, are relatively unnoticed as well. The mere existence of large populations of strange-looking nymphs, naiads and larvae of insects within the rubble and gravelly substrata of streams is fascinating. That so few people know of their existence is a pity. They have an important function in the aquatic food web and, as such, the structural composition of the macroinvertebrates will reflect changes in the quality or quantity of energy input.