The past decade has brought great strides in our understanding of adaptive immunity in neonatal mice. Although poor immune responses are commonly observed, it is now clear that mature function can be achieved by all arms of the adaptive immune system. An ever-increasing body of evidence indicates that the neonatal period of life is a unique developmental stage in which responses are highly plastic and dependent on the conditions of antigen exposure. This review focuses on our new understanding in mice and, where it is clear that related phenomena occur, in humans.