In vivo, only a species-specific number of ovarian follicles are selected to release their oocytes for fertilisation during each reproductive cycle. While many factors play a role in selecting the follicles for ovulation, the overriding factor is availability and follicular response to the circulating gonadotrophins, folliclestimulating hormone (FSH) and luteinising hormone (LH). Initially, oocyte growth is commensurate with follicle growth. In most species, oocytes are generally thought to have completed their growth phase by the early antral stage of follicular development. While gonadotrophins and steroids have been the focus of much research into follicular development, numerous other growth factors and cytokines have been identified as also having a role in oocyte developmental competence. Some of these factors affect the selection of follicles that proceed to the ovulatory stages while others directly enhance oocyte maturation. Another aspect of oocyte maturation is the epigenetic phenomenon of genomic imprinting.