Beetles make up the large, extremely diverse order Coleoptera of the class Insecta, and comprise the largest single group of animals on Earth. In fact, with approximately 400,000 described species in Coleoptera, beetles account for roughly one-quarter of all known species of animals. Estimates of the total number of beetle species range from 850,000 to 4,000,000 species. The rove beetle (Staphylinidae) and the weevil (Curculionidae) families of beetles alone contain approximately 60,000 and 40,000 species, respectively. Thus, the order Coleoptera, representing about 40 percent of the known insect species, contributes greatly to making the insects the largest class of the largest phylum—Arthropoda. Arthropods are thought to have evolved over 530 million years ago during the Early Cambrian, while the most primitive insect fossils date to the rocks of the Middle Devonian period, approximately 395 million years ago. Coleoptera are thought to have evolved in the Lower Permian about 270 million years ago, and were common even before the age of reptiles.

Beetles are found in virtually all climates and latitudes throughout the world, except at very high altitudes or in regions with extreme temperatures, such as the Antarctic. Most species of beetles occur in the tropics, but fewer individuals of a particular species are generally found in tropical regions rather than in temperate areas.

Beetles' success is due to at least three important characteristics. First, they undergo complete metamorphosis (egg, larva, pupa, adult), with larval and adult stages usually living in different places and eating different food. This division greatly expands the number of ecological niches and food available to them. Second, the front pair of wings is modified into a hard cover (elytra) that protects the soft body underneath. Third, most beetles have mouth parts capable of chewing a wide variety of solid foods. Some, however, have mouth parts modified for sipping sap and nectar.

The front pair of wings, modified into horny covers (elytra), hide the rear pair of wings and abdomen, and their inner edges appose each other, creating a straight line down the back of the insect. The elytra form a rigid, closely interlocking sheath that covers the mesothorax and metathorax, and most of the abdomen. (The name Coleoptera is derived from the Greek word koleos, meaning sheath.) The perfect alignment of the edges of the elytra form the characteristic, straight line that seems to split the back of the beetle, and gives these insects their common name (beetle, from the German word bheid, meaning to split).

Beetles are found on vegetation, under bark, stones, and other objects, as well as almost anywhere on or in the soil, rotting vegetation, dung, and carrion. They vary widely in size and appearance, and many have noteworthy behavior. Some beetles (e.g., Lampyridae) produce light, while others (Cerambycidae) can stridulate, that is, they can produce sound. Large beetles usually make a loud noise during flight, and some, such as the scarab beetles, have a bizarre physical form.