Fully grown mammalian oocytes as well as very early cleavage stage mammalian embryos contain inactive and atypical nucleoli that arc very distinct from nucleoli in stem or differentiated cells. It has been commonly accepted that these nucleoli serve as a storage site of material(s) from which, as the embryo develops, typical active nucleoli are gradually formed. For this reason, these atypical nucleoli are called the nucleolus precursor bodies (NPBs). It has also been demonstrated that the number and distribution of NPBs in pronuclei in human embryos can serve as a simple noninvasive indicator of further embryonic development. The invention of the enucleolation method as well as some recently published results clearly showed some additional roles of NPHs and clarified their Significance in normal embryonic development. In our chapter, these new results will be discussed especially in the context of their use in human assisted reproduction.