Beetles, known as Order Coleoptera, are the most diverse order in the animal kingdom, with the largest number of species. Over 350,000 beetle species have been described by science, with estimates putting the total number of species between 5 and 8 million. About 25% of all known life-forms and 40% of described insect species are beetles.

Beetles are placed in phylum Arthropoda (arthropods), class Insecta (insects). Like other insects, they have a hard shell, open circulatory system, and are primarily small invertebrates. What distinguishes beetles from other insects is their hardened forewings (from which the order gets its name: coleo = shield + ptera = wing).

Once, the biologist JBS Haldane was asked if he could say anything about God from his study of nature. Haldane replied, "He must have an inordinate fondness for beetles." Discovering and classifying new beetles is truly a challenge for biologists, and hundreds or thousands of new species are discovered every year, mostly in the tropical zone. When scientists leave out a net and shake trees in the tropics, thousands of species of beetles fall into the net. Beetles may be the easiest order of animal to find new species within.

Beetles range wildly in size. The smallest known beetle, Nanosella fungi, is 0.25 mm in length with a weight of 0.4 mg. The largest known beetle, iTitanus giganteus, exceeds 20 cm (8 in) in size and weighs over 100 grams. Beetles set other interesting records. For instance, the rhinoceros beetle species Dynastes hercules, can lift 850 times its own weight, a record left unchallenged until it was discovered that the tropical mite Archegozetes longisetosus could lift 1150 times its own weight. That's like an elephant that can carry 1150 other elephants on its back.

Because of their abundance and diversity, beetles are popular subjects for insect collections. They are dried, mounted, and put on display in hundreds of thousands of public and private collections throughout the world. It is through the study of beetles that many of the early post-Darwin biologists came to understand evolution better.