In North America, there are more than 8,600 species of insects associated with freshwater environments during some part of their lives. Just about anywhere you go on the planet, there is some kind of insect that will live in almost any place that stays wet for a week or so. Aquatic insects are important food for fish and waterfowl. They also play important roles in keeping freshwater ecosystems functioning properly. This article describes the biology and ecology of aquatic insects, the major groups of aquatic insects found in freshwater habitats, and how to observe them.