This chapter provides a contemporary overview of the key developmental processes that take place during the earliest stages of mammalian development, highlighting their sensitivity to environmental influences in a manner that can determine fertility, pregnancy outcome, and offspring health. In sexually mature adults, the process of ovarian folliculogenesis typically takes between 6 and 7 months, although the active period of growth is estimated to be around 12 weeks and witnesses a 400-fold increase in follicle volume. Preimplantation development can be characterized by three major transitions. The first transition concerns activation 172 of the embryonic genome, the timing of which varies between species. The second is compaction and the third transition is blastocyst formation. The success of assisted reproductive technology (ART) has, to a large extent, relied on the remarkable tolerance of mammalian gametes and cleavage-stage embryos to physical manipulations and alterations to their chemical environment.