Amphibian metamorphosis provides a model to elucidate the mechanisms underlying how vertebrates reconstitute a body plan and how the immune system develops during ontogeny. In Xenopus, T cells are expanded from the early developmental stages just after hatching. These T cells switch from larval-type in an easily tolerizable state into an adult-type having a potent immune responsiveness comparable to that of mammals. During metamorphosis, tadpoles exhibit morphological changes in skin that completely transforms from larval-type to adult-type. Only tail tissue behaves differently; it remains a larval-type tissue until it disappears at the end of metamorphosis. Thus, at metamorphic climax, four different types of cells co-exist in a tadpole body: larval tissue cells; adult tissue cells; larval immune cells; and adult immune cells. Based on the results showing that tadpole tail skin is rejected by syngeneic adult, it is proposed that the elimination of the larval tissue cells by the adult T cells that occurs during metamorphosis is immunologically mediated. Recent results indicate that the antigenic proteins expressed in the metamorphosing skin cells participate in the process of tail regression. This chapter describes how animals adjust and survive through such crises associated with large scale replacement of entire body cells.