1. Monotremes: These are the most primitive mammals and are characterized by laying eggs. They include the platypus and the four species of echidna.
2. Marsupials: These mammals give birth to live young, but the young are born in a very immature state and continue to develop in a pouch on the mother's body. Marsupials include kangaroos, koalas, and opossums.
3. Placental mammals: These mammals give birth to live young that are fully developed. They are the most diverse group of mammals and include humans, bats, whales, and mice.