A striking characteristic of marsupials is that they give birth to highly altricial young after a relatively short gestation period supported by a chorio-vitelline placenta. They complete much of their development within the pouch, dependent on a long and highly sophisticated lactation, producing milk of totally different compositions from adjacent mammary glands. A major difference in their reproduction 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 Jost paradigm that sexual differentiation depends only on hormones secreted by the testis when we demonstrated a number of hormone-independent sexual dimorphisms before the testicular differentiation. 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. Undoubtedly, the unique biology of marsupials provides novel perspectives for further understanding the evolution and control of successful mammalian reproduction and development.