Monotremes are mammals that lay eggs.

Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals.

They also are among the small number of mammalian species known to:

- be capable of electroreception (or the biological ability to perceive electrical impulses)
- not have communication between the right and left brain hemisphere
- have their urinary, defecatory, and reproductive systems all open into a single duct

The only surviving examples of monotremes are all indigenous to Australia and New Guinea. They are the platypus and the echidna.