Ground beetles are a large, cosmopolitan family of beetles, the Carabidae, with more than 40,000 species worldwide, around 2,000 of which are found in North America and 2,700 in Europe. As of 2015, it is one of the 10 most species-rich animal families. They belong to the suborder Adephaga. Members of the family are primarily carnivorous, but some members are herbivorous or omnivorous.

Although their body shapes and coloring vary somewhat, most are shiny black or metallic and have ridged wing covers (elytra). The elytra are fused in some species, particularly the large Carabinae, rendering the beetles unable to fly. The species Mormolyce phyllodes is known as violin beetle due to their peculiarly shaped elytra. All carabids except the quite primitive flanged bombardier beetles (Paussinae) have a groove on their fore leg tibiae bearing a comb of hairs used for cleaning their antennae.

Typical for the ancient beetle suborder Adephaga to which they belong, they have paired pygidial glands in the lower back of the abdomen. These are well developed in ground beetles, and produce noxious or even caustic secretions used to deter would-be predators. In some, commonly known as bombardier beetles, these secretions are mixed with volatile compounds and ejected by a small combustion, producing a loud popping sound and a cloud of hot and acrid gas that can injure small mammals, such as shrews, and is liable to kill invertebrate predators outright.

To humans, getting "bombed" by a bombardier beetle is a decidedly unpleasant experience. ^{[citation needed]} This ability has evolved independently twice, as it seems, in the flanged bombardier beetles (Paussinae), which are among the most ancient ground beetles, and in the typical bombardier beetles (Brachininae), which are part of a more "modern" lineage. The Anthiini, though, can mechanically squirt their defensive secretions for considerable distances and are able to aim with a startling degree of accuracy; in Afrikaans, they are known as oogpisters ("eye-pissers"). In one of the very few known cases of a vertebrate mimicking an arthropod, juvenile Heliobolus lugubris lizards are similar in color to the aposematic oogpister beetles, and move in a way that makes them look surprisingly similar to the insects at a casual glance.

A folk story claims that Charles Darwin once found himself on the receiving end of a bombardier beetle's attack, based on a passage in his autobiography. Darwin stated in a letter to Leonard Jenyns that a beetle had attacked him on that occasion, but he did not know what kind:

A Cychrus rostratus once squirted into my eye & gave me extreme pain; & I must tell you what happened to me on the banks of the Cam in my early entomological days; under a piece of bark I found two carabi (I forget which) & caught one in each hand, when lo & behold I saw a sacred Panagæus crux major; I could not bear to give up either of my Carabi, & to lose Panagæus was out of the question, so that in despair I gently seized one of the carabi between my teeth, when to my unspeakable disgust & pain the little inconsiderate beast squirted his acid down my throat & I lost both Carabi & Panagæus!

Common habitats are under the bark of trees, under logs, or among rocks or sand by the edge of ponds and rivers. Most species are carnivorous and actively hunt for any invertebrate prey they can overpower. Some run swiftly to catch their prey; tiger beetles (Cicindelinae) can sustain speeds of 9 km/h (5.6 mph) – in relation to their body length they are among the fastest land animals on Earth. Unlike most Carabidae, which are nocturnal, the tiger beetles are active diurnal hunters and often brightly coloured; they have large eyes and hunt by sight. Ground beetles of the genus Promecognathus are specialised predators of the cyanide millipedes Harpaphe haydeniana and Xystocheir dissecta, countering the hydrogen cyanide that makes these millipedes poisonous to most carnivores.

As predators of invertebrates, including many pests, most ground beetles are considered beneficial organisms. The caterpillar hunters (Calosoma) are famous for their habit of devouring prey in quantity, eagerly feeding on tussock moth (Lymantriinae) caterpillars, processionary caterpillars (Thaumetopoeinae) and woolly worms (Arctiinae), which, due to their urticating hairs, are avoided by most insectivores. Large numbers of the forest caterpillar hunter (C. sycophanta), native to Europe, were shipped to New England for biological control of the gypsy moth (Lymantria dispar) as early as 1905.