Animals are multicellular eukaryotic organisms that make up the biological kingdom Animalia. With few exceptions, animals consume organic matter, breathe oxygen, are able to move, reproduce sexually, and grow from a hollow ball of cells, the blastula, during the embryonic phase of development. More than 1.5 million living animal species have been described—about 1 million of which are insects—but it is estimated that there are more than 7 million animal species in total. Animals range from 8.5 micrometers to 33.6 meters in length and have complex interactions with one another and with their environment, as well as forming elaborate food webs. The study of animals is called zoology.

Most of the living animal species are classified in the Bilateria, a clade whose members have a bilaterally symmetrical body shape. The Bilateria includes protostomes and deuterostomes. Within the protostomes there are many groups of invertebrates, such as nematodes, arthropods, and molluscs, while the deuterostomes include echinoderms and chordates (including vertebrates). Life forms that are interpreted as ancient animals are classified within the Ediacaran biota that lived in the late Precambrian eon. Modern animal phyla formed clearly in the fossil record as marine species during the Cambrian explosion about 542 million years ago. 6,331 gene groups have been identified that belong to all living animals; these genes probably arose from a single ancestor that lived 650 million years ago.

Aristotle divided animals into those that have blood and those that don't. Carolus Linnaeus created the first hierarchical biological classification for animals in 1758 in his book, Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla in 1809. In the late 1800s, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now a synonym for from Animalia) and Protozoa, single-celled organisms that are no longer considered animals. In modern times, animal classification relies on sophisticated techniques, such as molecular phylogenetics, which are effective in demonstrating evolutionary relationships among animal taxa.

Humans use many other animal species for food, such as meat, milk, and eggs; for materials, such as leather and wool; as pets; and as working animals to be utilized for energy and used as a means of transportation. Dogs are used in hunting, while many land and water animals are hunted as sport. Animals have appeared in art since the earliest times and are a part of mythology and religion.