About 2,000 species of firefly are found in temperate and tropical environments. Many are in marshes or in wet, wooded areas where their larvae have abundant sources of food. These larvae emit light and are often called "glowworms", in particular, in Eurasia. In the Americas, "glow worm" also refers to the related Phengodidae. In many species, both male and female fireflies have the ability to fly, but in some species, females are flightless.

Fireflies, also known as glow worms, tend to be brown and soft-bodied, often with the elytra more leathery than in other beetles. Though the females of some species are similar in appearance to males, larviform females are found in many other firefly species. These females can often be distinguished from the larvae only because they have compound eyes. The most commonly known fireflies are nocturnal, though there are numerous species that are diurnal. Most diurnal species are nonluminescent, though some species that remain in shadowy areas can produce light.

Light production in fireflies is due to a type of chemical reaction called bioluminescence. This process occurs in specialised light-emitting organs, usually on a firefly's lower abdomen. The enzyme luciferase acts on the luciferin, in the presence of magnesium ions, ATP, and oxygen to produce light. Genes coding for these substances have been inserted into many different organisms (see Luciferase – Applications).

Firefly luciferase is used in forensics, and the enzyme has medical uses — in particular, for detecting the presence of ATP or magnesium. It has been speculated that Baroque painter Caravaggio may have prepared his canvases with a powder of dried fireflies to create a photosensitive surface on which he projected the image to be painted.

All fireflies glow as larvae. Bioluminescence serves a different function in lampyrid larvae than it does in adults. It appears to be a warning signal to predators, since many firefly larvae contain chemicals that are distasteful or toxic.

Light in adult beetles was originally thought to used for similar warning purposes, but its primary purpose is now thought to be used in mate selection. Fireflies are a classic example of an organism that uses bioluminescence for sexual selection. They have a variety of ways to communicate with mates in courtships: steady glows, flashing, and the use of chemical signals unrelated to photic systems.

Some species, especially lightning bugs of the genera Photinus, Photuris, and Pyractomena, are distinguished by the unique courtship flash patterns emitted by flying males in search of females. In general, females of the Photinusgenus do not fly, but do give a flash response to males of their own species.

Tropical fireflies, in particular, in Southeast Asia, routinely synchronise their flashes among large groups. This phenomenon is explained as phase synchronization and spontaneous order. At night along river banks in the Malaysian jungles (the most notable ones found near Kuala Selangor), fireflies (kelip-kelipin the Malay language or Bahasa Malaysia) synchronise their light emissions precisely. Current hypotheses about the causes of this behavior involve diet, social interaction, and altitude.

In the Philippines, thousands of fireflies can be seen all year-round in the town of Donsol (called aninipot or totonbalagonin Bicol). In the United States, one of the most famous sightings of fireflies blinking in unison occurs annually near Elkmont, Tennessee in the Great Smoky Mountains during the first weeks of June. Congaree National Park in South Carolina is another host to this phenomenon.

Female Photuris fireflies are known for mimicking the mating flashes of other "lightning bugs" for the sole purpose of predation. Target males are attracted to what appears to be a suitable mate, and are then eaten. For this reason the Photuris species are sometimes referred to as "femme fatale fireflies."

Many fireflies do not produce light. Usually these species are diurnal, or day-flying, such as those in the genus Ellychnia. A few diurnal fireflies that inhabit primarily shadowy places, such as beneath tall plants or trees, are luminescent. One such genus is Lucidota. These fireflies use pheromones to signal mates. This is supported by the fact that some basal groups do not show bioluminescence and, rather, use chemical signaling.

Phosphaenus hemipterushas photic organs, yet is a diurnal firefly and displays large antennae and small eyes. These traits strongly suggests pheromones are used for sexual selection, while photic organs are used for warning signals. In controlled experiments, males coming from downwind arrived at females first, thus male arrival was correlated with wind direction, indicating males' chemotaxis into a pheromone plume. Males were also found to be able to find females without the use of visual cues, when the sides of test Petri dishes were covered with black tape. This and the facts that females do not light up at night and males are diurnal point to the conclusion that sexual communication in P. hemipterus is entirely based on pheromones."