Macroinvertebrates are animals without a backbone ("invertebrate") that are large enough to be seen with the naked eye ("macro"). Freshwater macroinvertebrates spend part or all of their life cycle in flowing or standing waters; they include insects (fly nymphs and larvae, beetles and bugs), snails, mussels, leeches, worms, flatworms, slaters, and shrimps. Fly nymphs and larvae leave the water when they metamorphose and reach maturity, and spend their adult life on land. In many cases, the adult life stage lasts between several hours and a few days; while in contrast, the immature stage spent in water can last for one or two years.

Macroinvertebrates play an important role in the aquatic food web — they feed on algae and plant material, such as leaves, helping to break down organic matter. They also provide a source of food for other animals, such as fish, birds and mammals. Macroinvertebrates are good indicators of water quality because different species have different levels of tolerance to pollution. They also show integrated responses to pollution over time, rather than just at the time of sampling; and as such they are useful for assessing the health of freshwater ecosystems.

Eutrophication (over-enrichment with plant nutrients such as phosphorus and nitrogen) is a significant problem that affects the quality of fresh waters, leading to lower concentrations of dissolved oxygen in the water and less biodiversity. The main sources of these nutrients are discharges from wastewater treatment systems and run-off from agricultural land. Some freshwater macroinvertebrate species are more sensitive to this form of pollution than others and, by investigating which macroinvertebrate groups are present or absent and comparing their abundances, of these groups we can assess water quality. For example, stoneflies and most species of mayflies and caddisflies are very sensitive to organic pollution, whereas shrimps, worms, blackflies and midge (chironomid) larvae are more tolerant of organic matter and low oxygen levels.