The word chimera originally referred to a mythological beast with the head of a lion, the body of a goat, and the tail of a serpent, but it has come to mean any individual made up of the parts of more than one individual. Transplantation chimeras, which can be made in many species and sometimes between species, are formed when tissue is grafted from one embryo (the donor) to another (the host) and permitted to incorporate. This technique allows one to follow a specific group of cells (the graft) through a period of development and to determine the fates and locations of their progeny. To allow graft cells to be distinguished from the cells of the host after culture the graft is usually labeled with vital fluorescent dyes (see Note 1 ) or after fixation by using immunocytochemistry or a histological stain. Because chimeras can be made between a host and donor of the same stage (isochronic) or different stages (heterochronic), between similar tissues or regions (homotopic) or different tissues or regions (heterotopic), and between members of the same species (intraspecific) or different species (interspecific), minor variations in the technique allow one to investigate several parameters of development including cell fate, potency, commitment, evolutionary conservation of signals, and cell movement. For example, isochronic, homotopic grafting is used for fate mapping (and cell movement) studies (1 –3 ), heterotopic and heterochronic grafting yield information about competency and commitment (3 –11 ), and interspecific grafts can demonstrate evolutionary conservation of signaling pathways (12 –14 ) and may eventually lead to the analysis of the behavior of mutant mouse-embryo cells in chick-embryo hosts.