In February of 1997, Dolly the sheep was introduced (Wilmut et al., 1997) and the public subsequently was inundated with opinions about the power and potential of creating new animals from somatic cells. Dolly represented the most recent advance in genetic technology—the production of multiple individuals nearly genetically identical to an adult animal. In this technique, somatic cells from an appropriate tissue are grown in culture and their nuclei are injected into enucleated oocytes obtained from another individual of the same or a closely related species. After a further period of culture, the partially developed embryos are transferred into a foster mother. This technology is being developed rapidly for many species of interest (Table 2.1) and promises to become a rapid and efficient means of propagating domestic animals with desired traits, whether those are naturally derived and selected or genetically engineered (Betthauser et al., 2000; Lanza et al., 2001; Westhusin et al., 2001). The process often is referred to as “cloning” (see Chapter 1). The nuclear transfer technique was based on previous studies in frogs conducted during the previous 5 decades (Briggs et al., 1951; Prather et al., 1999), but, until Dolly, it was unclear whether nuclei from highly differentiated somatic cells could be reprogrammed to a pattern of gene expression suitable for directing normal development of a mammalian embryo.