The total number of oocytes (follicles) declines continuously after birth such that, by the time of menarche, only a quarter of a million follicles remains, or even fewer in girls prone to premature ovarian failure. In this chapter, attention has been drawn to ovarian ageing as an inexorable process, which has sometimes been simplistically compared to an hourglass. It focuses on the factors regulating the primordial-to-primary follicle growth transition. The number of follicles established at birth and the subsequent rate of loss determines the time of follicle depletion and hence menopause. The mechanism of the primordial-to-primary follicle transition has been evolutionarily conserved, as expected with a process that crucially affects fertility and the reproductive lifespan. The number of oocytes in the ovary is fixed early in life and not renewable in adulthood but a contrary view came to the fore, namely that oocytes are continually replaced from germline stem cells.