Invertebrates are animals without a backbone. 97% of all animal species are invertebrates, including all worms, insects, arachnids, crustaceans, mollusks, jellyfish, starfish, nematodes, sponges, and all microfauna. The smallest vertebrate, the female Paedocypris fish, is 7.9 mm long, so anything smaller than this is definitely an invertebrate. Genetically, a major difference between vertebrates and invertebrates is that invertebrates have only one cluster of Hox genes, while vertebrates have several. Humans have 4 Hox clusters. All of the 38 animal phyla except for a subset of one, Chordata, are invertebrates.

Vertebrates and invertebrates split evolutionarily between 525 and 520 million years ago, when eight early vertebrate animals appear in the fossil record, most notably the jawless fish Myllokunmingia. Prior to this, animals existed for at least 80 million years in an exclusively invertebrate form. Since then, vertebrates have been the world's most successful animals in terms of biomass, most notably crustaceans in the sea, arthropods on land, and nematodes throughout both.

In terms of size, vertebrates tend to have the advantage over invertebrates, although there have been some quite large invertebrates throughout history, including Mesonychoteuthis hamiltoni, the Colossal Squid, with a maximum size around 12–14 m (40–46 ft), the extinct eurypterids (sea scorpions), up to 2.5 m (8 ft) in size, Arthropleura (giant millipede), at 2.6 m (8.5 ft), and the griffinfly Meganeuropsis permiana, with a wingspan of 71 cm (2.3 ft). On average, however, invertebrates tend to be microscopic or insect-sized, while the typical vertebrate is fish or rodent-sized.

Because of their fast reproduction rates and comparative structural simplicity, some invertebrates, such as nematodes and fruit flies, are commonly used in science as model organisms. Much of what we know about genetics and biomechanics has come from detailed studies of invertebrates.

A subset of invertebrates, aquatic invertebrates visible to the naked eye, have been given the name "macroinvertebrates." The abundance and diversity of macroinvertebrates is considered an important indicator of the environmental state of an area. Macroinvertebrates are one of the most important links in the food chain above the level of microfauna and below the level of higher predators. Some macroinvertebrates predate on other macroinvertebrates, of course.