This chapter determines embryo viability in the human. Male and female gametes which unite to form each embryo are derived from a unique population of primordial cells that are designated to become the gametes: they develop into highly specialized gametes by the processes of spermatogenesis in the male and oogenesis in the female. Fertilization is a specialized example of cell-to-cell interaction. After fertilization the zygote divides by mitosis into a number of smaller cells called blastomeres. Cleavage may be considered a mitotic process, as found in adult somatic tissues, there is one important difference: in adult tissue the daughter cells grow following each division and are not able to divide again until they have achieved the original size of the parent cell. The metabolic requirements of the preimplantation embryo vary through the preimplantation period, and clear differences in metabolism are seen between pre- and post-compaction stages.