Mammalian oocytes acquire their intrinsic ability in a stepwise manner through ovarian folliculogenesis, ultimately becoming competent to undergo complete oocyte maturation at the final stage of the Graafian follicle. The fully-grown oocyte is tightly surrounded by compact layers of specialized granulosa cells (cumulus cells) known as the cumulus-oocyte complex. Oocyte maturation consists of the nuclear and the cytoplasmic maturation. Dynamic morphological changes such as cumulus expansion, chromosome alignment, and spindle formation are observed during the oocyte maturation. Mounting evidence that oocyte quality profoundly affects fertilization and subsequent embryo development drives the continued search for reliable predictors of oocyte developmental competence. It is necessary to understand the phenomenon and the molecular mechanisms active during oocyte maturation to obtain high-quality oocytes for in vitro maturation. In the present paper, we summarize the actions of the molecules that play key roles in meiotic progression and its control mechanism.