This chapter discusses the nature of interactions between oocyte and granulose cells and the triggering effect of luteinizing hormone (LH). The mammalian oocyte embarks on its first reduction division in prenatal life or during the early postnatal period. This division has a protracted and complicated prophase. Just before or shortly after birth, depending on the species, the germ cell reaches the stage of diplotene. By this time, the oocyte has doubled its DNA complement, the chromosomes have condensed, and homologous chromosomes have become paired and are linked by chiasmata permitting the exchange of paternal and maternal genetic information. In murid rodents, the chromosomes decondense and resume their transcriptive activity. In the adult, during each estrous cycle, a number of oocytes characteristic of the species complete their first reduction division, resulting in the abstriction of the first polar body shortly before ovulation. This resumption of meiosis and its progress to the metaphase of the second meiotic division refer to as ovum maturation. Completion of the second meiotic division, with extrusion of a second polar body, occurs only upon penetration of the oocyte by a spermatozoon.