Chordates include the vertebrates and a couple close cousins, the tunicates (sea squirts) and lancelets, primitive aquatic animals. In scientific classification, Chordata is one of 38 phyla of animals, one of the "big nine" along with Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, and Echinodermata. The precise origin of the chordates is unknown, but fish- or lancelet-like fossils from the Cambrian show that they existed at least 530 million years ago, during the so-called Cambrian explosion of biodiversity. The defining characteristics of chordates are a hollow dorsal nerve cord, a notochord, pharyngeal slits/pharyngeal pouch, an endostyle, and a post-anal tail.

The earliest chordates are found in the Chengjiang fauna of fossil beds in China, dated to 520-530 million years ago. Eight possible chordates are found in these fossils beds. The most famous is Myllokunmingia, a primitive, probably agnathan (jawless) fish, the first vertebrate to appear in the fossil record. It possessed a skull and what looks like a primitive notocord. Vaguely resembling a modern hagfish, Myllokunmingia was about 2.8 cm (1.1 in) long and 6 mm high, quite tiny. A related chordate is Haikouichthys ercaicunensis, another primitive fish-like animal. Another enigmatic fossil, Yunnanozoon lividum ("Livid Animal of Yunnan") is either a chordate or a hemichordate, a closely related phylum.

脊索动物后来成为所有动物门类中最成功的一个门类，在陆地和海洋上殖民，成为继节肢动物、线虫和软体动物之后的第四个最多样化的门类，拥有超过10万个物种。 脊索动物的主要创新是中央神经索，它演变成脊骨。 这为神经提供了一个指定的高速公路，使神经系统更加复杂和运动控制。 脊骨为动物不使用外骨骼而变得更加健壮提供了途径。

一些熟悉的脊索动物包括鱼类和鳗鱼，包括无颌鱼，如灯鱼、鲨鱼、鳐鱼和鳐鱼、两栖动物、爬行动物、鸟类和哺乳动物，包括人类。 脊索动物门中有一些已经灭绝的群体，包括刺鲨、被称为placoderms的装甲鱼、恐龙和许多古代四足动物，包括一些最早殖民陆地的动物，然后是最早殖民大陆内部的动物。