The presence of dead cells in the preimplantation mammalian embryo has been well described. Since Kerr et al. (1972), it has become apparent that these cells die by apoptosis, a form of programmed cell death. This review analyses the recent morphological and biochemical evidence that apoptosis play a role in early mammalian embryo development. Normal and apoptotic (i.e. fragmented) embryos express several apoptosis-related genes during mammalian preimplantation embryo development, with severe changes when apoptosis is activated; these findings support a model in which mammalian preimplantation embryo development is regulated by the ratio of pro- and -anti- apoptotic genes. Apoptosis may be a normal feature in human preimplantation development, even in vivo, and may play an active role in the developing embryo through the removal of genetically abnormal cells. Contrary to these beneficial effects, apoptosis may have detrimental effects if either the number of apoptotic cells or ratio of these cells to the normal cells is elevated. According to this value, embryos could either continue to develop or arrest.