By classical Linnaean classification, Mammals belong to class 'Mammalia'. Modern, phylogenetic classification too seems to put mammals in their own place in the tree of life, with the earliest mammals appearing some 250 million years ago. Mammal diversity increased much after the extinction of dinosaurs, some 66 million years ago, allowing mammals to become larger in size, and take a variety of forms. Mammals share several characteristics, in that they have a kind of layer of hair, are warm-blooded, and produce milk. Most mammals, placentals, give live birth, though a smaller number, the marsupials, grow their young in a pouch until they grow larger, and more defined. For where egg-laying mammals, monotremes, were once common, they have greatly diminished, in that the platypus and the echidna of Australia remain the only living Monotremes.

This collage contains a representative of each major mammal group. The zebra, 1), is a placental mammal, and gives live birth. The kangaroo, 2), is a marsupial, and grows its young in a pouch. The platypus, 3), is one of the most ancient mammals, and is a monotreme, and lays eggs.