Fireflies are 5 to 25 mm long. The head is hidden beneath a flattened pronotum, which is part of the thorax.
The soft elytra are normally brown or black, frequently with yellow or orange markings. There is a second pair of wings beneath the elytra. The females of many species are wingless or have very short wings.
The final two or three segments at the tip of the abdomen are generally luminous. They are a lighter shade. Depending on the species, both males and females or just one of the two sexes emits light.
Females are usually slightly larger than males.