The ovulatory follicle has the dual role of releasing a viable oocyte capable of fertilisation and the production of key endocrine signals that result in mating behaviour, the induction of the preovulatory LH surge and ovulation. Further, following ovulation the ovulatory follicle must be capable of forming a viable corpus luteum if pregnancy is to be maintained. Follicle growth is therefore a developmental process during which the follicle progressively acquires a number of properties, each of which is an essential prerequisite for further development. Failure to acquire these properties at the correct time and in an exact sequence will lead to failure of the developmental process and to the deterioration of the follicle through atresia and degeneration of the oocyte.