Trilobites are a group of extinct marine arthropods that first appeared around 521 million years ago, shortly after the beginning of the Cambrian period, and lived for nearly 300 million years until the end of the Permian period. They were highly diversified and geographically dispersed by the time they first appeared in the fossil record, and because of their wide diversity and easily fossilized exoskeleton, they left an extensive fossil record. Trilobites are known from more than 10,000 fossil species.

Trilobites had a distinctive three-fold longitudinal division of the dorsal exoskeleton into a central axis, flanked on either side by lateral (pleural) areas, which gives them their name. The trilobite body is divided lengthwise into three regions or tagmata: a head or cephalon, a middle region (thorax) composed of several to many articulated segments, and a tail plate called a pygidium, which consists of fused segments. The anterior region of the trilobite body is the head shield, or cephalon, which is made up of a series of fused segments. This commonly bears large, crescent-shaped compound eyes on the upper surface, which are thought to have had an incredible depth of field.

Trilobites occupied a variety of ecological niches. Some burrowed in sediment, while others crawled over the sea floor or swam in open water. Some were probably detritus feeders, scavengers, or predators. Other modes of life, such as filter feeding, have also been proposed. The great variety of body shapes and sizes indicate that trilobites occupied a variety of ecological niches.

The study of trilobite fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology, and plate tectonics. Geologists use trilobites in a variety of ways to help them understand how the Earth has developed. One use is in the relative dating and stratigraphical correlation of rocks.