Publisher Summary This chapter assists in identifying the small aquatic animals that one may find living in freshwater (nonmarine) habitats. Many of these organisms remain unseen due to their small size and secretive habits or are even ignored because identification is difficult. The invertebrates most commonly noticed are flashy or colorful (some crayfish, mussels, and beetles), good fishing bait (hellgrammites, caddisflies, mayflies), tasty (crayfish, shrimp, crabs), or an occasional painful nuisance now or later (toe biters, creeping water bugs, larval mosquitoes). Very few of our aquatic invertebrates can bite or pinch a human, and none can sting or poison you. Many of aquatic invertebrates are important indicators of water quality and habitat health. All the water bugs, snails, crayfish, mussels, and worms have important functions in processing organic material, controlling algae, and feeding fish, birds, turtles, otters, and raccoons.