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This review uses nutritional markers of normal and impaired development to address the question; what makes a viable mammalian preimplantation embryo? Resolution of this question is important to ensure the long‐term safety of embryo‐based biotechnologies in man and domestic animals, the optimisation of embryo production and culture conditions and the development of methods to select viable embryos for replacement. After considering the nutrition of embryos and somatic cells, and the phenomenon of caloric restriction, it is concluded that preimplantation embryo survival is best served by a relatively low level of metabolism; a situation achieved by reducing the concentrations of nutrients in culture media and encouraging the use endogenous resources. BioEssays 24:845–849, 2002. © 2002 Wiley Periodicals, Inc.