Ruminantia includes many of the well-known large grazing or browsing mammals: among them cattle, goats, sheep, deer, and antelope. All members of the Ruminantia are ruminants: they digest food in two steps, chewing and swallowing in the normal way to begin with, and then regurgitating the semi-digested cud to re-chew it and thus extract the maximum possible food value.

Not all ruminants belong to the Ruminantia. Camels and llamas are among the exceptions, a suborder known as Tylopoda. Also, there are a number of other large grazing mammals that, while not strictly ruminants, have similar adaptations for surviving on large quantities of low-grade food. Kangaroos and horses are examples.

The Tragulidae are the basal family in the Ruminantia.

The ancestral Ruminantia karyotype is 2n = 48 similar to that of cetartiodactyls.