Marsupials are only 6% of living mammals but have many characteristics that have taught us about the control of reproduction and development. A striking characteristic is that marsupials give birth to highly altricial young after a relatively short gestation period supported by a chorio-vitelline placenta. They deliver the smallest mammal at birth of any we know, at just 5 mg. They complete much of their development within the pouch, dependent on a long and highly sophisticated lactation. They can even produce milk of totally different compositions from adjacent mammary glands. Unlike all other mammals, their blastocysts have no inner cell mass, so the enigma of which cells are stem cells remains. They are also placental mammals, replete with genomic imprinting of certain genes in their fully functional placenta, contrary to popular dogma that marsupials don’t have a placenta! A major difference is the timing of their differentiation of sex, which all takes place post-natally, allowing easy manipulation of the process whilst the young is in the pouch. We discovered some unexpected findings that overturned the powerful Jost paradigm that sexual differentiation simply depended on hormones secreted by the testis when we demonstrated a number of hormone-independent sexual dimorphisms before the testicular differentiation. We now know that there are many other hormone-independent sexual dimorphisms, not only in mammals but also in birds. In addition, because of the post-natal gonadal development, we have been able to achieve testicular, prostatic and phallus sex reversal after treatment with oestrogen in vivo and in vitro. Finally, the long lactation, during which the composition of the milk changes dynamically to coordinate the specific growth requirement of the developing young has allowed us to examine the effects of fostering the young to a more advanced stage of lactation resulting in amazing growth acceleration and showing that the bioactives in the milk control the growth exquisitely regulating each stage of development. Marsupials may only be 6% of living mammals, but their unique biology provides novel perspectives for further understanding the evolution and control of successful mammalian reproduction and development. Acknowledgements: Supported by the Australian Research Council and the Australian National Health and Medical Research Council, and the efforts of many wonderful colleagues and students.