The organogenesis of the ovary encompasses the formation of a great variety of structures, both germinal and nongerminal. Primordial follicle (PF) formation is of the utmost importance because PFs are obligatory for the reproductive cycle and female fertility. The major events involved in PF formation are described. Areas that could benefit from more investigation are discussed. The working premise is that the number of PFs formed during normal ovary organogenesis varies from one female to the next (ranging from high to low), and that this variability is revealed by the timing of age-related infertility and the menopause. Implicit in this supposition is the concept that anything that alters the sequence of events involved in the process of PF development will have important consequences on female fertility and health.