The longhorn beetles (Cerambycidae), also known as long-horned or longicorns (whose larvae are often referred to as roundheaded borers), are a large family of beetles, with over 35,000 species described.

Most species are characterized by antennae as long as or longer than the beetle's body. A few species have short antennae (e.g., Neandra brunnea), making them difficult to distinguish from related families such as Chrysomelidae. "Cerambycidae" comes from a Greek mythological figure: after an argument with nymphs, the shepherd Cerambus is transformed into a large beetle with horns.

Longhorn beetles are found on all continents except Antarctica.

Other than the typical long antennal length, the most consistently distinctive feature of adults of this family is that the antennal sockets are located on low tubercles on the face; other beetles with long antennae lack these tubercles, and cerambycids with short antennae still possess them. They otherwise vary greatly in size, shape, sculpture, and coloration. A number of species mimic ants, bees, and wasps, though a majority of species are cryptically colored. The titan beetle (Titanus giganteus) from northeastern South America is often considered the largest insect (though not the heaviest, and not the longest including legs), with a maximum known body length of just over 16.7 cm (6.6 in).

Larvae are 0.5–22 cm (0.20–8.66 in) long, elongate in shape and lightly sclerotised. The prothorax is often enlarged and the sides of the body have lateral swellings (ampullae). The head is usually retracted into the prothorax and bears well-sclerotised mouthparts. Larval legs range from moderately developed to absent. The spiracles are always annular.

All known longhorn beetle larvae feed on plant tissue such as stems, trunks, or roots of both herbaceous and woody plants, often in injured or weak trees. A few species are serious pests. The larvae, called roundheaded borers, bore into wood, where they can cause extensive damage to either living trees or untreated lumber (or, occasionally, to wood in buildings; the old-house borer, Hylotrupes bajulus, is a particular problem indoors).

Many longhorns locate and recognize potential hosts by detecting chemical attractants, including monoterpenes (compounds released en masse by woody plants when stressed), ethanol (another compound emitted by damaged plant material), and even bark beetle pheromones. Many scolytine weevils share the cerambycid's niche of weakened or recently deceased trees; thus, by locating scolytinids, a suitable host can likely be located as well. The arrival of cerambycid larvae is often detrimental to a population of scolytinids, as the cerambycid larvae will typically either outcompete them with their greater size and mobility, or act as direct predators of them (this latter practice is less common, but has been observed in several species, notably Monochamus carolinensis). Cerambycids, in turn, have been found to play a role in attracting other wood-borers to a host. Borgemeister, et al. 1998, recorded that cerambycid activity in girdled twigs released volatiles attractive to some bostrichids, especially Prostephanus truncatus. A few cerambycids, such as Arhopalus sp., are adapted to take advantage of trees recently killed or injured by forest fires by detecting and pursuing smoke volatiles.

Adults of Lamiinae, most Lepturinae and some Cerambycinae also feed. Adults of Parandrinae, Prioninae and Spondylidinae do not feed. In those taxa with feeding adults, common foods are nectar, pollen, fruit and sap exudates. Some (mainly Lamiinae) feed on bark, plant stems, needles or developing cones. Roots are consumed by larvae and sometimes also adults of soil-dwelling Dorcadion. The genus Leiopus is known to feed on fungi. Lastly, the genus Elytroleptus is unusual in having carnivorous adults, which prey on lycid beetles.