Genomic imprinting (parent of origin gene expression) is an epigenetic phenomenon that results in monoallelic gene expression. It is widespread in eutherian mammals, and also occurs in marsupials. The host defence hypothesis suggests that imprinting evolved from existing mechanisms within the cell that acted to silence foreign DNA elements such as retrotransposons that insert into the genome. However, the changes to the mammalian genome that accompanied the evolution of imprinting have been hard to define. A large number of imprinted genes are found in the mammalian placenta but the brain is also an important site. However, until recently this data has been largely derived from study of just two of the 5000 or so species of mammals. Marsupials have been separated from eutherian mammals for around 160 MYA, so are ideal mammals in which to test the origin and control of genomic imprinting. Although imprinting evolved before the marsupial-eutherian split, the mechanisms have continued to evolve in each lineage to produce differences between the two groups. As yet there is no evidence for genomic imprinting in the egg-laying monotremes, although these mammals also form a short-lived placenta and transfer nutrients from mother to embryo. Imprinting was therefore not necessary for the evolution of the placenta and its role in nutrient transfer, but does appear to be associated with the evolution of viviparity, at least in mammals. Intriguingly, we have now demonstrated that there are imprinted genes in the marsupial; mammary gland. Recent analyses of imprinted gene clusters in marsupials and monotremes show that there have been.an accumulation of repeats, especially LTRs and DNA elements, in therian imprinted genes and gene clusters but not in monotremes. This provides strong support for the host defence hypothesis and may have been a potential driving force in, the development of mammalian genomic imprinting.