The Craniata, or craniates, include all animals having a skull (or cranium, hence their name), be it cartilaginous or bony. A skull is a box of hard tissues which encloses the brain, olfactory organs, eyes, and internal ear. Craniates comprise all fishes - including such jawless fishes as hagfishes and lampreys - amphibians, reptiles, birds and mammals, including Man. The earliest known undisputed craniates are jawless fishes which lived 480 millions years ago. Their evolutionary history took place first in the sea, then in fresh waters. Some craniates, the tetrapods or four-legged vertebrates, became terrestrial and arose about 370 millions years ago from fish ancestors. Now, the majority of the craniate species are represented by one group of fish, the actinopterygians, and the tetrapods. Other craniate groups (jawless fishes, sharks and chimeras, the coelacanth, lungfishes) are considerably depauperate, by comparison with their past diversity.

The Craniata fall into two major clades, the Hyperotreti, or hagfishes, and the Vertebrata. Since the early nineteenth Century, and until recently, the hagfishes were regarded as the sister-group of lampreys (Hyperoartia). The two groups were gathered into the clade Cyclostomi (see below: Discussion of phylogenetic relationships). However, lampreys share with jawed vertebrates, or Gnathostomata, a large number of morphological and physiological characteristics that occur neither in hagfishes, nor in non-craniate chordates (cephalochordates and tunicates). There is, therefore, a rather broad consensus over the theory that hagfishes are the sister-group of the Vertebrata (lampreys and gnathostomes).