Mayflies are a group of winged insects classified under Order Ephemeroptera, which means "short-lived wing" in Greek. Mayflies are famous for their short lifespans: ranging from a day to as short as 30 minutes. This makes them the shortest-lived of all known animals, including all known microorganisms, whose lifespan is almost always longer than a day. For an animal to be considered "short-lived" in the zoology community, it has to live less than about a week.

Mayflies superficially resemble a cross between dragonflies and crickets, to both of which they are related. Mayflies spend months to years in an undeveloped form called a nymph. Mayflies of a given population tend to all mature simultaneously, covering every available surface and being ubiquitous for their very short lifespans. They live just long enough to reproduce and die. By contrast, most other insects live for a few weeks at the least.

There are about 2,500 species of mayfly known worldwide, with 630 species found in North America. Like dragonflies and damselflies, mayflies are classified in an ancient group called the Paleoptera. This group is characterized by the presence of wings but no ability to fold them back, which most other insects possess. Most insects fall under Neoptera, meaning "new winged insects," which are known to be a monophyletic group (descending from a common ancestor). The Paleoptera might not be a monophyletic group, but simply what remains when the Neoptera are subtracted. Most members of Paleoptera are fossils, with relatively few extant species.

The mayflies are known as animals very sensitive to pollution, meaning that where they are found, the water is very clean. Mayfly naiads primarily live in streams under rocks, but a few live on lakes. These are among the most prolific, and occur in such concentrations that their simultaneous growth into adult form has been detected using doppler radar. Lake Erie is famous for its concentration of mayflies.