The aim of developmental biology is to understand how an egg converts itself into a complete organism through the processes of cell differentiation, morphogenesis and size regulation. The principles that have emerged over recent decades include the constancy of the genome in nearly all cells of an individual, the existence of stem cells in many organs and the overwhelming importance of signalling between cells for the determination of their fate. These and other characteristics of development are discussed here in relation to the prospect of achieving cell and tissue correction or replacement with the help of nuclear transplantation and signalling factors. Nuclear transplantation offers a one–step procedure for generating multipotent embryo cells from the cells of an adult tissue such as skin. It should be possible to proliferate the resulting cells as can be done for mouse embryonic stem cells. Embryo cells can be made to differentiate in many directions by exposing them to various agents or to different concentrations of a single factor such as the transforming growth factor β class signalling molecule activin. The possibility of a cancerous condition being acquired during these experimental manipulations can be guarded against by transfecting cells with a conditional suicide gene. Thus it may be possible to generate replacement cells or tissues from an adult human for transplantation back to the original donor, without the disadvantage of any genetic incompatibility.