The process whereby a fertilized egg develops into more than 200 different cell types and forms a new individual is both intriguing and fundamental to developmental biology. The systematic pathway of cellular differentiation is generally thought to confer a restriction whereby differentiated cells lose the capacity to change into other cell types. However, the substitution of nuclear material from one cell type (egg) with that of another, in a process known as nuclear transfer or cloning, can reverse this restriction by removing epigenetic modifications to the chromatin structure. The ability to clone new individuals using this procedure was achieved some 30 years ago in amphibia in an effort to understand cellular differentiation. More recently, with the advent of improved embryo micromanipulation techniques, the technology has been applied to domestic and laboratory animals.