Marsupials are excellent objects for studies on developmental processes in all mammals including humans. Marsupials are very immature at birth and undergo most of their development in a pouch where they can be manipulated in a variety of ways without affecting the mother. Most of these studies are on systems which largely mature before birth in eutherian mammals and are consequently difficult to investigate. Attention is also drawn to certain features peculiar to adult marsupials: e.g., they continue to grow throughout adult life, valuable for studies on growth mechanisms, and furthermore the composition of marsupial milk changes radically through lactation, most important in studies of hormonal regulation of milk composition and secretion.