Monotremes are prototherian mammals of the order Monotremata. They are one of the three main groups of living mammals, along with placentals (Eutheria) and marsupials (Metatheria). Monotremes are typified by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts compared to the more common mammalian types. In addition, they lay eggs rather than bearing live young, but, like all mammals, the female monotremes nurse their young with milk.
Monotremes have been considered members of Australosphenida, a clade which contains extinct mammals from the Jurassic and Cretaceous of Madagascar, South America and Australia, though this is disputed.
The only surviving examples of monotremes are all indigenous to Australia and New Guinea although there is evidence that they were once more widespread, as Monotrematum is known from the Paleocene of South America. The extant monotreme species are the platypus and four species of echidnas. There is currently some debate regarding monotreme taxonomy.
The name monotreme derives from the Greek words μονός (monós 'single') and τρῆμα (trêma 'hole'), referring to the cloaca.